Add armrest tag to amenity=bench preset

[bompstable]

See https://josm.openstreetmap.de/ticket/23694

[bompstable]

I've previously used a combo for this but after experimentation, I've decided I prefer to use a simple checkbox for this, but have set the text to "Armrests (one or more)" to make the usage clearer and to avoid text such as "Armrest(s)" which can be difficult to translate.

Here's what the current version using checkboxes looks like:

And here's the previous version using a combo: