Add measurement field type for `directionalCombo`

[tordans]

We have a good discussion on what this field should look like for users and what it needs to look like technically in #15
And there is a working schema already in #198

During the last community chat the Martin asked if we need measurement capabilities for the directionalCombo field type.

Here are some cases, where a field type measurement would improve the directionalCombo:

• maxspeed:forward <km/h>
• sidewalk:right:width
• cycleway:right:width

There are different ways to resolve this…

• Ignore it for now; we don't have those fields, yet – but we want to have them in the near future
• Add a measurement property but not a measurement field type; allow editors to add measurement validations and transformations on all field types that have this additional definition. Right now, the measurement definitions would only apply to the number and directionalCombo field type (full list). Are there tagging practices that would require other field types?
• Make the directionalCombo a flag that add capabilities to an existing field. Right now we would only allow and use this on the combo field type. Are there tagging practices that would require other field types? — That does not really resolve the issue, though, because we still need to add measturement to combo then.
• Add a directionalMeasurement field type that is a mix of the two concepts.
• …

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Ping @k-yle @1ec5 @tyrasd FYI and for comments. I extracted this into a separate issue to keep the threads separate.

[1ec5]

Add a measurement property but not a measurement field type; allow editors to add measurement validations and transformations on all field types that have this additional definition. Right now, the measurement definitions would only apply to the number and directionalCombo field type (full list). Are there tagging practices that would require other field types?

This seems reasonable at a glance as part of the next set of breaking changes. It would be a breaking change because we’d need to migrate the existing roadheight and roadspeed–typed fields to number without affecting the ability to enter units. Apart from these types and the two you’ve mentioned, I don’t think any of the other types would be relevant to this feature.