Add remove_unused_with assist

[CPardi]

Hi, I've had a look at implementing the unused with assist described on #39. The changes include adding a remove_unused_with function that:

• Retrieves diagnostics at the cursor position.
• Checks for an UnusedWith diagnostic overlapping the with keyword range; if none is found, the assist does not apply.
• Removes the entire with expression, including the with keyword, its expression, the semicolon, and any trailing trivia.

Additionally, I have added some tests and updated the code_action.md document.