Enable implicit type assertions for array types

[jfirebaugh]

In #5698, I had to write:

        "text-font": [
          "array",
          "string",
          [
            "get",
            "text-font"
          ]
        ]

I wanted to be able to write:

        "text-font": [
          "get",
          "text-font"
        ]

Where the type assertion is inserted automatically by the implementation, as it would be for a scalar-typed style property.