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# Description
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Rel: #14429, #16079

Finishes up a TODO in the assignment type checking. 

- For regular assignment operations (only applies to `mut`), type
checking is now done using `type_compatible` (which is what `let` uses)
- This allows some mutable assignments to work which weren't allowed
before

Before:
```nushell
let x: glob = "" 
# => ok, no error
mut x: glob = ""; $x = ""
# => Error: nu::parser::operator_incompatible_types
# => 
# =>   × Types 'glob' and 'string' are not compatible for the '=' operator.
# =>    ╭─[entry #6:1:19]
# =>  1 │ mut x: glob = ""; $x = ""
# =>    ·                   ─┬ ┬ ─┬
# =>    ·                    │ │  ╰── string
# =>    ·                    │ ╰── does not operate between 'glob' and 'string'
# =>    ·                    ╰── glob
# =>    ╰────

let x: number = 1
# ok, no error
mut x: number = 1; $x = 2
# => Error: nu::parser::operator_incompatible_types
# => 
# =>   × Types 'number' and 'int' are not compatible for the '=' operator.
# =>    ╭─[source:1:20]
# =>  1 │ mut x: number = 1; $x = 2
# =>    ·                    ─┬ ┬ ┬
# =>    ·                     │ │ ╰── int
# =>    ·                     │ ╰── does not operate between 'number' and 'int'
# =>    ·                     ╰── number
# =>    ╰────
```

After:
```nushell
let x: glob = ""
# ok, no error (same as before)
mut x: glob = ""; $x = ""
# ok, no error

let x: number = 1
# ok, no error (same as before)
mut x: number = 1; $x = 2
# ok, no error
```

- Properly type check compound operations. First checks if the operation
(eg. `+` for `+=`) type checks successfully, and then checks if the
assignment type checks successfully (also using `type_compatible`)
- This fixes some issues where the "long version" of a compound
assignment operator would error, but the compound assignment operator
itself would not

Before:
```nushell
mut x = 1; $x = $x / 2
# => Error: nu::parser::operator_incompatible_types
# => 
# =>   × Types 'int' and 'float' are not compatible for the '=' operator.
# =>    ╭─[entry #15:1:12]
# =>  1 │ mut x = 1; $x = $x / 2
# =>    ·            ─┬ ┬ ───┬──
# =>    ·             │ │    ╰── float
# =>    ·             │ ╰── does not operate between 'int' and 'float'
# =>    ·             ╰── int
# =>    ╰────

mut x = 1; $x /= 2
# uh oh, no error...

mut x = (date now); $x = $x - 2019-05-10
# => Error: nu::parser::operator_incompatible_types
# => 
# =>   × Types 'datetime' and 'duration' are not compatible for the '=' operator.
# =>    ╭─[entry #1:1:21]
# =>  1 │ mut x = (date now); $x = $x - 2019-05-10
# =>    ·                     ─┬ ┬ ───────┬───────
# =>    ·                      │ │        ╰── duration
# =>    ·                      │ ╰── does not operate between 'datetime' and 'duration'
# =>    ·                      ╰── datetime
# =>    ╰────

mut x = (date now); $x -= 2019-05-10
# uh oh, no error... (the result of this is a duration, not a datetime)
```

After:
```nushell
mut x = 1; $x = $x / 2
# => Error: nu::parser::operator_incompatible_types
# => 
# =>   × Types 'int' and 'float' are not compatible for the '=' operator.
# =>    ╭─[entry #5:1:12]
# =>  1 │ mut x = 1; $x = $x / 2
# =>    ·            ─┬ ┬ ───┬──
# =>    ·             │ │    ╰── float
# =>    ·             │ ╰── does not operate between 'int' and 'float'
# =>    ·             ╰── int
# =>    ╰────

mut x = (date now); $x -= 2019-05-10
# => Error: nu::parser::operator_incompatible_types
# => 
# =>   × Types 'datetime' and 'datetime' are not compatible for the '-=' operator.
# =>    ╭─[entry #11:1:21]
# =>  1 │ mut x = (date now); $x -= 2019-05-10
# =>    ·                     ─┬ ─┬ ─────┬────
# =>    ·                      │  │      ╰── datetime
# =>    ·                      │  ╰── does not operate between 'datetime' and 'datetime'
# =>    ·                      ╰── datetime
# =>    ╰────
# =>   help: The result type of this operation is not compatible with the type of the variable.
```

This is technically a breaking change if you relied on the old behavior
(for example, there was a test that broke after this change because it
relied on `/=` improperly type checking)

# User-Facing Changes
<!-- List of all changes that impact the user experience here. This
helps us keep track of breaking changes. -->
* Mutable assignment operations now use the same type checking rules as
normal assignments
* For example, `$x = 123` now uses the same type checking rules as `let
x = 123` or `mut x = 123`
* Compound assignment operations now type check using the same rules as
the operation they use
* Assignment errors will also now highlight the invalid assignment
operator in red


# Tests + Formatting
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Make sure you've run and fixed any issues with these commands:

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> **Note**
> from `nushell` you can also use the `toolkit` as follows
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> use toolkit.nu # or use an `env_change` hook to activate it
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> ```
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Adds some tests for the examples given above

# After Submitting
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N/A