[ty] Display docs for matching parameter when hovering over the name of an argument passed by keyword. (#25283)

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## Summary

<!-- What's the purpose of the change? What does it do, and why? -->

This makes it so that hovering over the name of an argument passed by
keyword shows the matching parameter label and documentation rather than
the inferred type of the argument value:

```py
def test(ab: int):
    """my cool test

    Args:
        ab: a nice little integer
    """
    return 0

test(ab=123)
      ^
```

````txt
(parameter) ab: int
---------------------------------------------
a nice little integer
````

Two additional notes on this new behaviour:
- For a parameter without any documentation, we will show just the
parameter label: `(parameter) ab: int`.
- We will also show the documentation for a variadic keyword parameter
(i.e., `**kwargs`) when the argument name maps to that type of
parameter.

By contrast, nothing changes when hovering over the argument value: it
will continue to show the inferred type of the value:

```py
value = 123

def test(ab: int):
    """my cool test

    Args:
        ab: a nice little integer
    """
    return 0

test(ab=value)
          ^
```

````txt
Literal[123]
````

Closes https://github.com/astral-sh/ty/issues/1350.
Closes https://github.com/astral-sh/ty/issues/3538.

## Test Plan

Please see included tests.

<!-- How was it tested? -->