syntax highlighting broken for `.*eex` files

After the recent change that [removed ftdetect](https://github.com/elixir-editors/vim-elixir/pull/560), syntax highlighting is no longer working for `.*eex` files (`.heex`, `.leex`, etc). It appears that the default filetype detection marks these files as e.g. `ft=heex` instead of `ft=eelixir`, as the previous `ftdetect` plugin did.

# Actual

```
set ft?
filetype=heex
```


# Expected

If I run `set ft=eelixir`, I get this:


However, if I first run `let b:eelixir_subtype = 'heex'`, I get this:


Not sure which of these is correct.

```
$ nvim --version
NVIM v0.7.0-dev+1348-g524f6294a
Build type: Release
LuaJIT 2.1.0-beta3
Compiled by a80323765@USC02F834HMD6N

Features: +acl +iconv +tui
See ":help feature-compile"

system vimrc file: "$VIM/sysinit.vim"
fall-back for $VIM: "/usr/local/Cellar/neovim/HEAD-524f629/share/nvim"

Run :checkhealth for more info
```

Review the change that removed the ftdetect plugin and reproduce detection for .heex, .leex, and other .*eex files in Neovim. Compare the resulting ft=heex behavior with ft=eelixir and the eelixir_subtype=heex case; done means these files receive the intended syntax highlighting automatically.

Written by the indexing model from the issue text.