Make file path in request body relative to the project

[MoritzKn]

The code_completion_service.cpp seems to expect a relative path:

if (path == "res://" || !path.begins_with("res://"))
	return Result();

Without this change the server was always responding with a 404 status and the autocompletion didn't work.

[MoritzKn]

@neikeq could you take a look on this please?

[kubecz3k]

@MoritzKn Neikeq is working on c# support like a mad man, you will need to forgive him any potential delays :)

[MoritzKn]

No stress, it's fine. I just thought maybe this PR got forgotten.