Support `line-progress` for `line-offset` property

Nobody has claimed this yet.

## Motivation

I'm trying to draw a subway map: lines are getting together in bunches and branch off.
For example, here C-line (blue) and G-line (ligh-green) branch off:

While they are in bunch, I use `line-pattern`:

But then, I need to make a smooth transition from a pattern to 2 separate lines, which then would be centered normally.

More complicated example where one bunch branches of into 2 other bunches:

## Design Alternatives

Split line into smaller chunks, assign an `offset` property to each segment separately.

```json
"line-offset": [
"case",
["has", "offset"],
["to-number", ["get", "offset"]],
0
]
```

There is a visible staircase effect when number of segments is low, especially on a small zoom:

## Design

Ideally, I will need to set something like this:

```json
"line_offset": [
"interpolate",
["linear"],
["line-progress"],
0, 2.5, // where 2.5 is some magic number for when line is in a bunch
1, 0
]
```

Unfortunately, `line-progress` can only be used in the `line-gradient` property.
https://docs.mapbox.com/mapbox-gl-js/style-spec/expressions/#line-progress

I wonder how feasible it is to implement it for `line-offset` as well.
Or, perhaps, there are better ways to solve my problem.

Start with the existing `line-gradient` support and the style-spec expression documentation linked in the issue, then determine how `line-progress` is evaluated for `line-offset`. Done means a style can use the proposed expression with `line-offset` and produce a smooth transition along the line without requiring segmented features.

Written by the indexing model from the issue text.