Feature request: expression for string.split(<separator>)

Nobody has claimed this yet.

## Motivation

*Underlying problem: The MVT format does not support array properties*

I want to send 48 numbers (between 0 and 1) along with every feature. These values represent congestion in electrical grid lines over 48 hours. I want to have a time slider to pick the hour, and style every feature according to their individual congestion at that hour.

*A workaround for this that I've used in OpenLayers is to ***use a string property, which contains all the values separated by semicolon. They are then split up in the style function***, and the "current congestion" (according to the slider) is used to style each line individually.*

It seems like this is not easily doable in Mapbox GL, which is why I'm making this feature request.

## Design Alternatives

It might be the case that there is some good alternative to this approach that I'm not seeing (that does not severely affect performance or scalability).

## Design

Proposition: implement the following expression:
```js
["split", separator_string, string]: array
```

Example:
```js
["split", ";", "1;0.5;0.33;0.101;0.2"]
-> ["1", "0.5", "0.33", "0.101", "0.2"]
```

The behavior should essentially be the same as the following (javascript):
```js
// ";" is the separator_string:
"1;0.5;0.33;0.101;0.2".split(";")
// ["1", "0.5", "0.33", "0.101", "0.2"]
```

Start by locating the expression evaluator and the existing expression test coverage for array-producing expressions. Implement the proposed split(separator_string, string) behavior, then verify that the example returns the expected string array and that edge cases match JavaScript split semantics.

Written by the indexing model from the issue text.