Addon manager plugin support

Explain the feature

The Lua Language Server supports plugins that allow users to write custom Lua scripts that the language server will then run to output a modified file. As of right now, the addon manager will download the plugin.lua file, but it will not tell the server to use it. There should be some easy (and safe) way for users to enable a plugin that they have installed.

Are there any potential drawbacks from adding the feature?

There are some serious security concerns with downloading a script off the internet and running it automatically on the user's computer. While the addons being installed should be reviewed and trusted before being merged into LLS-addons, there is always the chance of something slipping through.

We could ask the user to review the code themselves before approving the request to enable it - but at the end of the day, the safest option will always be to not run code created by other users.

Start by tracing the addon manager's handling of the downloaded plugin.lua file and review LuaLS's runtime.plugin setting documentation. Clarify the approval and safety requirements for enabling an installed plugin, then define how completion will be verified without automatically running unreviewed code.