add categorical expression

[josxha]

This pull request adds support for categorical expressions.

Example JSON snippet for a categorical expression:

"icon-image": {
      "property": "seamark:small_craft_facility:category",
      "type": "categorical",
      "stops": [
        [
          {
            "zoom": 0,
            "value": "showers"
          },
          "Showers"
        ],
        [
          {
            "zoom": 0,
            "value": "launderette"
          },
          "Launderette"
        ]
      ]

[josxha]

@greensopinion Added the requested changes. Should be ready for another review.

[deckerst]

@josxha is this PR abandoned?

[josxha]

@deckerst yes, it's currently abandoned.