Handle inmemory files better

[kofno]

VSCode will send an inmemory:// file for new files are files that are opened as part of a git diff.

Two problems arise:

1. Crystal doesn't handle require's from these files, since it can't recognize the in memory path, so error checking them will report false positives.
2. We aren't cleaning up the reported errors when the inmemory file is closed.

To solve #1, we may want to only run the parser analyzer over inmemory files (and offer them completion services, when those are available).

To solve #2, we should send a clean diagnostic report whenever we close an inmemory file.

[kofno]

Fixed in 940f891