Add registration handlers for IL sub-instructions in PluginCommand

[mostobriv]

What is the feature you'd like to have?
There is already register_for_xxx_level_il_instruction handlers, it would be awesome to have same thing for expressions (or sub-instructions, as all of them is just an instruction types), to be able to distinguish cases when plugin is triggered when cursor is focused on token instead of whole line. Currently no matter where focus is, the ILInstruction provided to handler would be the whole instruction on the line.

Are any alternative solutions acceptable?
Well, currently UIAction can do all of this, but IMHO in that way there is unnecessary legwork to do.