Support elixir and phoenix heex files in neovim (#1450)

In neovim, the newer Phoenix HTML templates are [identified as `heex`
files](https://github.com/neovim/neovim/blob/2b421d518faec9e1b0a6d1c46026d2f13f4fb73f/runtime/lua/vim/filetype.lua#L571).
Additionally, there can be HTML code within `.ex` files (via `~H""`),
which both neovim and VSCode identify as `elixir`.

Author: patryk-s

packages/tailwindcss-language-service/src/util/languages.ts

@@ -8,13 +8,15 @@ export const htmlLanguages: string[] = [
   'django-html',
   'edge',
   'ejs',
+  'elixir',
   'erb',
   'gohtml',
   'GoHTML',
   'gohtmltmpl',
   'haml',
   'handlebars',
   'hbs',
+  'heex',
   'html',
   'HTML (Eex)',
   'HTML (EEx)',