Incremental synchronization support

[lnicola]

Spawned off #3758.

Some LSP clients (ahem, Gnome Builder) don't respect our choice for full text synchronization, and it wouldn't hurt to reduce the amount of text transferred.

The text change notification comes in here. params.content_changes contains all changes, but right now we only process one of them. Before passing the changes to the VFS layer, we need to convert from a line/column to byte offset representation. The latter is implemented as the TextRange type, and a conversion method is available here.

[matklad]

Another potential complication is that we store files with normalized line endings (\n), so we need to account for that. I think just normalizing the text from TextDocumentContentChangeEvent would do the trick, as ranges are ending-agnosic.