Add assets to the token provider

[adam-coster]

Using Ctrl+P to find GameMaker stuff is a little unpleasant, since we usually don't care about YY files but also filenames in general aren't very useful (e.g. we often want to find a function defined in a script, but don't really care about the script).

The VSCode extension implements a token provider for globals (via Ctrl+T), but this doesn't include assets since they aren't defined in code. This makes it really annoying to quickly open up an object, for example.

To resolve this, we need to add assets to the token provider:

• For objects, add one entry per event where the location is the event's GML file
• For scripts, add one entry to the start of the GML file
• For everything else, add one entry to the YY file