Implement textDocument/didSave

[radeksimko]

It appears the LSP spec doesn't allow for this method to be disabled via capabilities, which means it's probably required (?), so we should implement it: https://microsoft.github.io/language-server-protocol/specifications/specification-current/#textDocument_didSave

[radeksimko]

This was practically implemented as part of #336 although the validation is opt-in.

[ghost]

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