[plugin] start document listening when all clients are ready

[akosyakov]

What it does

• fix [plugin] initial document deltas are missing sometimes #6319: start document listening when all clients are ready

Otherwise events for already opened document can be missed.

How to test

• install vscode go extension
• install some go project, i.e. go get github.com/petar/GoLLRB/llrb
• configure preference to use go LS and see its tracing information:

{
  "gopls.trace.server": "verbose",
  "go.useLanguageServer": true
}

• open go project and open a file
• open gopls output view
• check that didOpen, didChange, didClose events are delivered whenever you do a corresponding action
• make sure to test it after reload with preserved editors
• also test it with reconnections by restarting the backend

Hint: you can clean the output view when an event should be at the top

Review checklist

• as an author, I have thoroughly tested my changes and carefully followed the review guidelines

Reminder for reviewers

• as a reviewer, I agree to behave in accordance with the review guidelines

[benoitf]

it makes sense 👍