fix: resolve the right formatter for each file

[doorgan]

#804 added a formatter cache, but it fails in the following scenarios:

Closing the first formatted file:

1. Open file a.ex and format it
2. Open file b.ex and format it
3. Close file a.ex
4. Try to format b.ex
5. It fails with :no_formatter in the logs

This is because a.ex and b.ex will both point to the same entry in the cache. Removing a.ex will remove that path AND its entry, so the next time we request a formatter for b.ex, it will not find the old entry and the cache will return :no_formatter

This sharing of the formatter entry causes a second issue. Because they share the same entry and formatter function, any failure to format b.ex will point to a.ex, as that's the filename closed over by the formatter function in the entry cache:

1. Open file a.ex and format it
2. Open file b.ex, introduce a syntax error, and format it
3. A formatter error will be logged(not notified to the user though), but it points to a.ex instead of b.ex

This PR fixes both issues by changing the cache to track one formatter function per path, ensuring no formatter function is shared, and that closing a document won't break formatting for other files.

Note: it is very annoying that the formatter works by creating a bespoke function for each file, and that the formatter plugins don't seem to inherit the formatter options if we override them to set the right :file for each path, so this is what we have for now.