build: add luals check

This automatically downloads and uses the correct luals binary for the
currently used system. `make luals` will run luals on all lua files in
`runtime`.

We download lua-language-server manually instead of relying on
contributors downloading it on their own (like with stylua) as
lua-language-server is updated frequently which may cause unnecessary
friction. Therefore, we download a pinned version of luals which we then
can manually bump when needed. This can be re-evaluated if luals becomes
more stable in the future.

Currently this is not run when using `make lint` since cmake style "file
caching" doesn't seem possible at the moment. This is because checking a
single file doesn't seem to work.

Work on https://github.com/neovim/neovim/issues/24563.

.github/workflows/test.yml

@@ -57,6 +57,10 @@ jobs:
         name: stylua
         run: cmake --build build --target lintlua-stylua
 
+      - if: success() || failure() && steps.abort_job.outputs.status == 'success'
+        name: luals
+        run: cmake --build build --target luals
+
       - if: success() || failure() && steps.abort_job.outputs.status == 'success'
         name: luacheck
         run: cmake --build build --target lintlua-luacheck