VSCode does not recognize heredocs as markdown, and highlight/lint appropriately

### Is this the right repo?

I believe this is the right repo, as the issue, as the issue I'm observing is related to syntax highlighting in VSCode.

### Environment

* Elixir & Erlang versions (elixir --version): Erlang/OTP 23 [erts-11.2.2] [source] [64-bit] [smp:10:10] [ds:10:10:10] [async-threads:1]

Elixir 1.12.0 (compiled with Erlang/OTP 23)

* VSCode ElixirLS version: 0.13.0
* Operating System Version: darwin 21.2.0

### Troubleshooting

- [X] Restart your editor (which will restart ElixirLS) sometimes fixes issues
- [X] Stop your editor, remove the entire `.elixir_ls` directory, then restart your editor

### Crash report template

No Crash

### Description

When working in elixir files with `@moduledoc` and `@doc` attributes, they are not recognized as markdown. VSCode doesn't lint them appropriately, and there's no obvious way to provide appropriate previews of the files.

I'm not sure if this is a new feature, or a bug.

I've reviewed some of the code that appears to define this. I suspect this is defined in:

https://github.com/elixir-lsp/vscode-elixir-ls/blob/master/elixir-language-configuration.json

and

https://github.com/elixir-lsp/vscode-elixir-ls/blob/master/syntaxes/elixir.json

However, I'm quite uninitiated here, so I don't know what I'm looking at. There appear to be some accomodations for `@moduledoc` and `@doc` attributes, but I'm not sure if they are correct.