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.
When a file has no extension but contains a shebang (e.g., `#!/bin/bash`),
filetype detection now reads the first 10 lines from disk and uses
`vim.filetype.match({ filename, contents })` for content-based detection.
This enables syntax highlighting for files like `build` scripts that rely
on shebang detection, even when the file isn't open in a buffer.
Detection order:
1. Existing buffer's filetype (already implemented in #69)
2. File content (shebang/modeline) - NEW
3. Filename extension only
Also adds `filetype on` to test helpers to ensure `vim.g.ft_ignore_pat`
is set, which is required for shell detection.
