[feature] array_foreach expression function - add counter variable

[pathmapper]

Original motivation is here to combine values from multiple array fields after OGR GML import for label expressions, e.g.

array_foreach( "field_1", @element || ' ' || "field_2"[@counter] )

but there are other use cases for sure.

Test added and function help updated.

[Gustry]

Good idea.
Can you add a note that the counter starts at 0 ?
Usual, but better mentioning than guessing ;-)

[pathmapper]

Thanks @Gustry, done in 38be39e.

[qgis-bot]

@pathmapper

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@pathmapper
This pull request has been tagged as requiring documentation.

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@pathmapper
A documentation ticket has been opened at qgis/QGIS-Documentation#8582
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