filesystem: Support incremental updates

[radeksimko]

The current implementation updates file content in full (from the first to last byte), which may cost more resources than necessary. This is usually not noticeable if users open smaller files, but may become an issue for users with bigger files.

Keeping files small helps maintainability and is a good practice in general (and hence expected in most cases), but we may not want to hurt the UX for the edge cases with big files too much either.

Incremental updates are part of the LSP and both server and client have to opt-in via relevant capabilities:
https://microsoft.github.io/language-server-protocol/specifications/specification-current/#textDocument_synchronization_sc

[ghost]

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