fix(parser): detect filetype from file content (shebang/modeline)

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.

spec/helpers.lua

@@ -2,6 +2,8 @@ local plugin_dir = vim.fn.getcwd()
 vim.opt.runtimepath:prepend(plugin_dir)
 vim.opt.packpath = {}
 
+vim.cmd('filetype on')
+
 local function ensure_parser(lang)
   local ok = pcall(vim.treesitter.language.inspect, lang)
   if not ok then