Diagnostics only execute after changes are made

[mateusabelli]

The code for diagnostic checking run only upon changes on the document. This behavior is alright but it should also run diagnostics when the file is opened instead of waiting for a document change.

This a sample of the code that is being used for diagnostics in server/server.py

@server.feature(TEXT_DOCUMENT_DID_CHANGE)
async def did_change(ls: LanguageServer, params: DidChangeTextDocumentParams):
  [...]