Set *.theme files as the PHP filetype

This wasn't working with vim-polyglot enabled, as per
https://github.com/neovim/neovim/issues/23522, so I'm removing at - as
least temporarily.

after/plugin/filetype.lua

@@ -0,0 +1,5 @@
+vim.filetype.add {
+  extension = {
+    theme = "php",
+  },
+}