Improve completions inside of a pipeline

…tion

add os-specific installation instructions to front page

closes #9111 

# Description

this pr improves parsing of values with units (`filesizes`, `durations`
and any other **future values**) by:

1. allowing underscores in the value part
```nu
> 42kb          # okay
> 42_sec        # okay
> 1_000_000mib  # okay
> 69k_b         # not okay, underscores not allowed in the unit
```

2. improving error messages involving these values
```nu
> sleep 40-sec

# before
Error: nu::parser::parse_mismatch

  × Parse mismatch during operation.
   ╭─[entry #42:1:1]
 1 │ sleep 40-sec
   ·       ──┬──
   ·         ╰── expected duration with valid units
   ╰────

# now
Error:
  × duration value must be a number
   ╭─[entry #41:1:1]
 1 │ sleep 40-sec
   ·       ─┬─
   ·        ╰── not a number
   ╰────
```

3. unifying parsing of these values. now all of these use one function

# User-Facing Changes

filesizes and durations can now have underscores for readability

… 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
       ╰────
    ```