Plugin support for hover

[TwitchBronBron]

Moves the getHover functionality into the plugin API so plugins can participate in providing hover support.

Notable changes:

• add new beforeProvideHover, provideHover, afterProvideHover plugin events.
• Plugins can push 0 or more Hovers to the event.hovers collection.
• All hovers are deduped, and then merged into a single markdown string each separated by a single newline. This allows multiple plugins to provide hovers for a given position. If plugins don't want to step on each other, they can first check that the event.hovers is empty.
• modify the plugin event pipleline to short-circuit if the handler returns false. This is useful for onGetHover because only a single plugin may contribute a hover, so the first one that contributes needs to indicate they did so.
• move BrsFile getHover functionality into the BscPlugin. I didn't change the core logic, I just copied the code and made it work within the new object.