Feature: more advanced `for` loops

Is your feature request related to a problem? Please describe.
Working with frequencies and sweeping frequencies is a really fun way to make sound with the micro:bit V2.

The 'for loop' is an ideal construct for this, as it lets you make some good sounds with very little code.

Describe the solution you'd like
I'd like a block that lets the user set the start and increment value for the for loop index variable.

I think this could make sense as a new block

for <index> from <start> to <end> with step <step> translating to:

for (let index = start; index <= end; index+=step) {
}

Describe alternatives you've considered

The user can always use maths in the loop to do this, but for many students this greatly increases the complexity of the code required for simple sound effects.

We've done whole activities around playing with a 'frequencySweep' function which could be entirely replaced with a for loop

stinging together a range of these with different start, end, step and delays would make cool sounds.