feat(runtime)!: enable filetype.lua by default (#19216)

* revert to filetype.vim by setting `g:do_legacy_filetype`
* skip either filetype.lua or filetype.vim via `g:did_load_filetypes`

(Running both is no longer required and therefore no longer supported.)

src/nvim/testdir/test_filetype_lua.vim

@@ -1,3 +0,0 @@
-let g:do_filetype_lua = 1
-let g:did_load_filetypes = 0
-source test_filetype.vim

src/nvim/testdir/test_legacy_filetype.vim

@@ -0,0 +1,4 @@
+let g:do_legacy_filetype = 1
+filetype on
+
+source test_filetype.vim