Parse errors for def, let and alias

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# Description
Fixes #10300 , where using variables didnt work with `ucp` as it was
only expecting a `Expr::FilePath`.

Before: (from the issue)
```
❯ ucp -r $var $folder
Error:   × Missing file operand
   ╭─[entry #40:1:1]
 1 │ ucp -r $var $folder
   · ─┬─
   ·  ╰── Missing file operand
   ╰────
  help: Please provide source and destination paths
```
Now:
```
`ucp -r $var $folder`
# success
```

Also added the test to ensure its working:) . Oh, and I tweaked again
slightly the messages on two tests because now the whole `path` is
printed rather than `a`. Say:
```
#before
`cp a a` --> 'a' and 'a' are the same file 
# now
`cp a a` --> /home/current/location/a and /home/current/location/a are the same file
```
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# User-Facing Changes
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# Tests + Formatting
Don't forget to add tests that cover your changes.

Make sure you've run and fixed any issues with these commands:

- [X] `cargo fmt --all -- --check` to check standard code formatting
(`cargo fmt --all` applies these changes)
- [X] `cargo clippy --workspace -- -D warnings -D clippy::unwrap_used`
to check that you're using the standard code style
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make sure to [enable developer
mode](https://learn.microsoft.com/en-us/windows/apps/get-started/developer-mode-features-and-debugging))
- [X] `cargo run -- -c "use std testing; testing run-tests --path
crates/nu-std"` to run the tests for the standard library
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… commands to utilize that (#16414)

# Description

- Implemented `FromValue` for `std::time::Duration`.
- It only converts positive `Value::Duration` values, negative ones
raise `ShellError::NeedsPositiveValue`.
- Refactor `job recv` and `watch` commands to use this implementation
rather than handling it ad-hoc.
- Simplified `watch`'s `debounce` & `debounce-ms` and factored it to a
function. Should make removing `debounce-ms` after its deprecation
period ends.
- `job recv` previously used a numeric cast (`i64 as u64`) which would
result in extremely long duration values rather than raising an error
when negative duration arguments were given.

# User-Facing Changes

Changes in error messages:
- Providing the wrong type (bypassing parse time type checking):
  - Before
    ```
    Error: nu::shell::type_mismatch

      × Type mismatch.
       ╭─[entry #40:1:9]
     1 │ watch . --debounce (1 | $in) {|| }
       ·         ──────────┬─────────
       ·                   ╰── Debounce duration must be a duration
       ╰────
    ```
  - After
    ```
    Error: nu:🐚:cant_convert

      × Can't convert to duration.
       ╭─[entry #2:1:9]
     1 │ watch . --debounce (1 | $in) {|| }
       ·         ──────────┬─────────
       ·                   ╰── can't convert int to duration
       ╰────
    ```
- Providing a negative duration value:
  - Before
    ```
    Error: nu:🐚:type_mismatch

      × Type mismatch.
       ╭─[entry #41:1:9]
     1 │ watch . --debounce -100ms {|| }
       ·         ────────┬────────
       ·                 ╰── Debounce duration is invalid
       ╰────
    ```
  - After
    ```
    Error: nu:🐚:needs_positive_value

      × Negative value passed when positive one is required
       ╭─[entry #4:1:9]
     1 │ watch . --debounce -100ms {|| }
       ·         ────────┬────────
       ·                 ╰── use a positive value
       ╰────
    ```