neovim: add lua language server lint checks to CI

[fidgetingbits]

Atm warnings from vscode triggered by sumneko.lua plugin (using lua language server) don't fire in CI. This should ideally added via .pre-commit so people that run that locally can also see the errors in advance.

[pokey]

we would also ideally find a way to have a single source of truth for the lua language server config. Today it's all in .vscode/settings.json, which won't be read by CI