Add remove_unused_binding assist

[CPardi]

Hi,

I've had a go at implementing an assist to remove bindings for the UnusedBinding liveness check described on #39. If you have time to take a look that would be great, thanks.

The changes include adding a remove_unused_binding function which does the following steps

1. Retrieves the diagnostics at the cursor and if UnusedBinding is not present then return
2. Assume the cursor is at a let binding and attempt to delete it
   • I started from the pack_bindings implementation and removed the inner bindings logic to get the TextRange to delete.
3. If the previous fails, assume the cursor is at a pattern binding and attempt to delete it
   • the code checks to see if there is a comma after the pattern name
     • If so, then it is deleted along with the trivia after
     • If not, the preceding comma and trivia is deleted
4. If both let and pattern binding cases fail then return without any edits. (I have assumed that there are only these two cases)

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