Support iteration of array subscripts within GCode expressions

[dc42]

Is your feature request related to a problem? Please describe.

RRF 3.5.0-beta.4 accepts this (iterating implicitly through move.extruders):

m409 k"move.extruders[].stepsPerMm"

giving result:

{
    "key": "move.extruders[].stepsPerMm",
    "flags": "",
    "result": [
        1,
        2,
        3,
        4
    ],
    "next": 0
}

However, it does not accept this:

echo move.extruders[].stepsPerMm

It should preferably perform the iteration implicitly in an expression such as the above, yielding an array result.