implement textDocument/codeAction

[taylorplewe]

Implement textDocument/codeAction (spec) to show a list of possible Commands for a given range of code.

Possible command ideas that come to mind:

• "Convert to macro": highlight a block of code, create a new .macro (somewhere) containing that block of code, replace the original block of code with a call to the macro (and possibly, find all other places in the codebase containing that same block of code and replace with the macro call)
• ^ same but with a .proc?
• if the developer types an instruction or tries to use an addressing mode that is not compatible with the default target processor or the target processor they specified in their config file, but is compatible with another target processor, prompt the user to change their target processor (e.g. the developer specified "6502" as their target processor; they type the instruction "stz"; they get prompted to change their target processor to either "65C02", or "65802" or "65816")