fix(highlight): include treesitter injections

Problem: highlight_treesitter only iterated captures from the base
language tree (trees[1]:root()). When code contained injected
languages (e.g. VimL inside vim.cmd()), those captures were never
read, so injected code got no syntax highlighting in diffs.

Solution: pass true to parse() to trigger injection discovery, then
use parser_obj:for_each_tree() to iterate all trees including
injected ones. Each tree gets its own highlights query looked up by
ltree:lang(), and @spell/@nospell captures are filtered out.

spec/helpers.lua

@@ -12,6 +12,7 @@ local function ensure_parser(lang)
 end
 
 ensure_parser('lua')
+ensure_parser('vim')
 
 local M = {}