Internally swap implementation of vertical text-anchor to use text-offset

Nobody has claimed this yet.

## Motivation

Prerequisite to: #10755

`text-anchor` and `text-offset` both move characters vertically, relative to lines, producing the artifacts below:

However, `text-offset` is applied in [`placeGlyphAlongLine`](https://github.com/mapbox/mapbox-gl-js/blob/53fda413221c183c1006881f378401173c251325/src/symbol/projection.js#L490), where something can be done about it. `text-anchor` is applied during [text shaping](https://github.com/mapbox/mapbox-gl-js/blob/53fda413221c183c1006881f378401173c251325/src/symbol/shaping.js#L749-L757), without knowledge of the projected geometry. As a result, even though they have an identical effect, the distribution of characters can only easily be improved for `text-offset`.

Thus I propose, for the specific case of text along lines, to ignore `text-anchor: top|bottom` and instead pass the vertical offset through to glyph placement, similar to `text-offset`, so that it may be applied in a location where the distribution of characters can be fixed.

Such a PR would be an internal change as a prerequisite for #10755, with no external effect.

My primary concern is that there is symbol behavior I'm not yet aware of which would prevent this from being a simple internal change.

## Design Alternatives

I'm not aware of a reasonable alternative apart from simply not addressing the character spacing problem for `text-anchor`.

## Design

I believe there are two challenges:

1. pass the vertical offset resulting from text-offset through to `placeGlyphAlongLine`
2. scale the offset by the correct font size

cc @mapbox-gl-js @mapbox/gl-native

Start by reading the text-anchor handling in src/symbol/shaping.js and text-offset handling in src/symbol/projection.js, especially placeGlyphAlongLine. Trace how vertical offsets reach glyph placement and determine the correct font-size scaling. Done means text-anchor top|bottom is internally routed through glyph placement for text along lines without external behavior changes.

Written by the indexing model from the issue text.