Improve docs/supplemental files for plugins

[Nerixyz]

While trying to add docs for plugins on the wiki, I noticed some issues in the plugin docs:

• The JSON schema didn't allow $schema as a key.
• The TypeScript types contained the wrong event type.
• register_callback wasn't documented at all.
• No Lua metadata for plugins was present. LuaLS seems to be the most common language server.

[pajlada]

Looks good to me 👍
Would be cool if the docs/plugin-meta.lua file could be auto-generated since it's a bit of a duplicate of LuaAPI.hpp

[Mm2PL]

Would be cool if the docs/plugin-meta.lua file could be auto-generated since it's a bit of a duplicate of LuaAPI.hpp

I can work on that. In the mean time this PR is a good step towards that.