need to introduce a VariableDefinition type

[sumeet]

instead of having the assignment_id and name inside of the Assignment itself. this opens itself to

• being able to highlight ONLY the variable definition
• being able to edit variable names in things that aren't variables like anon funcs, for loops, and match statements
• i think totally removes the need for match.make_variant_id (phew, that was scary)
• simplifies lookback code for Variables, instead of all those different cases, only look for VariableDefinitions