File api

[TwitchBronBron]

Moves file handling into plugin-land so plugins can contribute custom file types. (Migrated from #387 )

Notable changes:

• added plugin event onProvideFile (and beforeProvideFile/afterProvideFile events too). Plugins can add files to the .files array of the event.
• All files found in the files array that exist on disk will be included in the program. Files not handled by plugins and bsc will be loaded as AssetFile instances, and are copied as-is. (this will help support the custom tree view we wanted to add in vscode but couldn't because non xml and brs files were missing)
• Defined a new interface called File that desribes the important parts of a file. This makes most file properties optional, so plugin-contributed files only add what they actually implement.
• add beforeFileRemove and afterFileRemove events, deprecate beforeFileDispose and afterFileDispose events.
• add beforeFileAdd and afterFileAdd events that are triggered after the provide events for every file that plugins create.