Support specifying globals inside Lua files

Currently, when using this language server, you get warnings for global variables:

if someGlobalVariable then -- Undefined global `someGlobalVariable `. Lua Diagnostics.(undefined-global)
  print("wow!")
end

The way to squelch them is to add the globals to the VSCode settings. But this is too much of a sledgehammer, as we might only want the globals to be enabled in this specific file. Thus, we should also add a feature so that end-users can specify the globals in-line:

-- We specify a list of globals at the top of the file.
-- @global someGlobalVariable 

someCode()

if someGlobalVariable then -- No warning.
  print("wow!")
end

Start by locating the undefined-global diagnostic handling and the existing VSCode global-variable setting in the language server. Trace how Lua comments are parsed, then define and test the inline @global syntax so the named global is accepted only in that file without changing project-wide settings.