Completion and highlighting for builtin modules

[bfoersterling]

general goal

Adding support for ansible builtin modules.

changes

highlighting

In the current syntax highlighting you cannot see if the builtin modules typed into the editor actually exist.
Also the modules debug and shell are highlighted in different colors.

With support for the builtin modules you can instantly detect typos:

completion

After the pattern ansible.builtin. autocompletion can be used with an omni function.
(ctrl + x + o)

[pearofducks]

This looks like a great addition, thanks for getting a PR together!

Two quick things:

• Can you add a quick note about it in the README?
• Is the highlight group you're creating for this user-overrideable?
  • If not, is it trivial to make it so?
  • If so, can you also add this to the README?

(I should be much faster to respond now, I was out of town on a trip last week)