Allow M99 to return a named variable and value

[benagricola]

Implements #1038, for after v3.6.

I thought I would get this submitted as a potentially very simple (and in my opinion elegant) implementation that allows a value to be returned from a macro file, through the use of M99 with a V and R value:

; Return variable and value provided by caller
M99 V{param.V} R{param.R}

This will create, or override the local variable named by V with the value of R in the calling macro or file.

My inspiration for this was trying to implement a new RS485 communications protocol mcode that functions like the new M261.1, which takes a V variable name and puts the returned data into that variable.

I like this approach of setting a configurable local variable in the calling block nesting as it means the data will be garbage collected properly, and by allowing the return to write to an existing variable it is possible to call a macro in a loop (using it like a function for example) without having to name the variable differently every loop iteration.

NOTE: This is not implemented as part of FileMacroCyclesReturn as this function is called from outside M99 as well.