Add variables to better customize plugin directories

[code-asher]

• Still loads all plugins from the plugins directory as before
  • But now PLUGIN_DIR will set that directory instead of being used to specify an individual plugin
  • This is the directory I envision being used for distributing code-server with plugins or where plugins would be placed for running instances of code-server in production by users
• PLUGIN_DIRS can be used to set multiple individual plugins if you don't want to move them into a single directory
  • Really only intended for development

Lemme know if we wanna change this up, this is just based off the VS Code plugin structure (except they use command line flags and not environment variables).

Closes #2122.

[code-asher]

Default plugin dir should actually use XDG I think. Thoughts? Having it inside code-server is weird, except maybe for built-in plugins. So maybe we should load both.

[code-asher]

I went ahead and did that.

[code-asher]

Should note this isn't backwards compatible with the previous method so maybe it would be better to maintain the old PLUGIN_DIR behavior and use different variables for the new stuff (or use command line flags instead). If this gets merged and released in time though I suppose it's all good.

[nhooyr]

Can we use CS_PLUGIN_PATH instead?

Where plugin path consists of colon separated directories in which plugins are loaded from.

And defaults to: $HOME/.config/code-server/plugins:/etc/code-server/plugins.

[code-asher]

CS_PLUGIN_PATH is a much better name.