initialize cache on startup

[shellderp]

we may as well create the keyword-scanning cache as soon as the plugin is loaded (in a background thread) this way the user won't feel lag the first time they use keyword lookup, autocomplete etc

potential issues:

• project might not be loaded on start-up, so we don't know what to cache. additionally, as far as I know there is no way to know when a new folder is added other than querying for it.
• do we cache if project-keyword-scanning is user-disabled?
• on project open? is that possible? (when a directory is added)