Allow expressions as elements in array output values

mapbox-gl-js version: v0.44.0

It would be great if I could use expressions in each element of an array value for properties like text-font and *-translate.

Lets take text-offset for example. The output value is a 2-element array. As a literal, can be written like this:

[0,1]

Lets imagine a hypothetical situation where I wanted to offset along the y axis based on the existence of a property in my data. This would be a useful feature if I had icons for some labels but not for others. I can write this expression like so, and it's valid:

[
  "case",
  ["has", "icon"],
  ["literal", [0, -10]],
  ["literal", [0, 0]]
]

If I have more than one case where I want to adjust my y axis value (say, to check for the existence of 'big-icon' and 'small-icon'), the syntax gets even more unwieldy:

[
  "case",
  ["has", "icon-small"],
  ["literal", [0, -10]],
  ["has", "icon-large"],
  ["literal", [0, -20]],
  ["literal", [0, 0]]
]

It would be amazing if the following was valid:

[
  0,
  [ "case", 
    ["has", "icon-small"],
    -10
    ["has", "icon-large"],
    -20,
    0
  ]
]

Now, that probably won't work because of the way the expression syntax is designed. Could a to-array expression get me what I want?

[
  "to-array",
  0,
  [ "case", ["has", "icon"], -10, 0]
]

This would be a really valuable feature for many properties, and particularly text-font, where usually users will have a universal fallback font, but would want to use an expression to dictate what the primary font is. It would also lead to a more user-friendly experience in Studio!

Start by reviewing the expression syntax and the handling of array-valued properties such as text-offset, text-font, and *-translate. Compare the existing literal-array and case examples with the proposed element-wise expressions or to-array form. Done means a supported, documented syntax works for these properties and the relevant expression behavior is covered by tests.