fix: Support omnifunc's alternative return type (dict)

[wookayin]

As per :help complete-functions, a complete function may return a list
or a dictionary { words = <list>, refresh = 'always' }. This was not
previously recognized, silently resulting in "no completion results".

An example of such omnifunc is vim.treesitter.query.omnifunc().

In order to return more information than the matching words, return a Dict
that contains the List.  The Dict can have these items:
	words		The List of matching words (mandatory).
	refresh		A string to control re-invocation of the function
			(optional).
			The only value currently recognized is "always", the
			effect is that the function is called whenever the
			leading text is changed.
Other items are ignored.

Note: workaround for vim.treesitter.query.omnifunc by monkey-patching until this gets merged:

-- Workaround /~https://github.com/hrsh7th/cmp-omni/pull/10
vim.treesitter.query.omnifunc = function(...)
  local ret = require('vim.treesitter._query_linter').omnifunc(...)
  return type(ret) == "table" and ret.words or ret
end

[wookayin]

Note: #4 also includes this kind of fix /cc @hrsh7th @akh2008

[wookayin]

/cc @hrsh7th Bumping this up.

[wookayin]

@hrsh7th Bumping this up again.

[wookayin]

@hrsh7th

[ribru17]

@hrsh7th Please merge 🥲