fix(filetype): normalize full path before matching #32227

Problem:
On Windows, treesitter query files are not recognized as such when
opened from inside their directory, because the full path returned from
fnamemodify(_, ':p') contains backslashes, while the filetype patterns
expect forward slashes.

Solution:
Normalize the result of fnamemodify(_, ':p') before trying to match it
to filetype patterns.

Co-authored-by: Justin M. Keyes <justinkz@gmail.com>

test/functional/lua/filetype_spec.lua

@@ -173,6 +173,23 @@ describe('vim.filetype', function()
 
     eq(buf, api.nvim_get_current_buf())
   end)
+
+  it('matches full paths', function()
+    mkdir('Xfiletype')
+    command('lcd Xfiletype')
+    eq(
+      'Xfiletype',
+      exec_lua(function()
+        vim.filetype.add({
+          pattern = {
+            ['.*/Xfiletype/Xfilename'] = 'Xfiletype',
+          },
+        })
+        return vim.filetype.match({ filename = 'Xfilename' })
+      end)
+    )
+    rmdir('Xfiletype')
+  end)
 end)
 
 describe('filetype.lua', function()