Introduce colonCombo field type

[tordans]

There are tagging practices that extend the key with a number in order to simulate an array of values.

Similar solutions (Docs):

• The semiCombo uses semicolons to simulate the array:

   foo=bar;baz
  

• The multiCombo encodes the value (from a defined list of values) into the key:

  foo:bar=yes
  foo:baz=yes
  

In the case of the panoramax key, the semiCombo is limiting the number of images to about 6 due to each panoramax value and and the max length of the field. See openstreetmap/id-tagging-schema#1344 (comment)

The multiCombo is not used for this kind of tagging where the value is something unique.

Instead MapComplete adds numbers to the key:

panoramax=123,
panoramax:0=234
panoramax:1=345

Historically, there where other cases where this :1 key postfix was used but I am not sure if any still exist that are relevant for the tagging schema. Some where fallbacks from TIGER imports AFAIR and some where used for the deprecated parking schema.

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I am not sure if this one panoramax use case is worth adding another combo field type. But lets use this issue to find out…