Add text-vertical-align style spec property

Nobody has claimed this yet.

## Motivation

This idea came out of https://github.com/mapbox/mapbox-gl-js/issues/8484. A `text-vertical-align` property is analogous to the [`vertical-align` CSS property](https://developer.mozilla.org/en-US/docs/Web/CSS/vertical-align) and would enable users to align text of varying sizes along a baseline, middle or top. If there were a compelling use case for it, it may be possible to add "sub" and "super" options as well.

This would require #191 to be implemented first so that GL JS has access to the glyph metrics in order to determine the correct baselines.

## Design Alternatives

At present, I don't believe there's a way to achieve this. Setting the baseline probably requires fiddling with font size and possible even the font family to find something that works which is not particularly useful. I can't really think of a way to align text to the top or middle.

A possible alternative is implementing text padding which could allow a user to move the text around with positive and negative margins. This is more complex for the user however and would still require a lot of trial and error. The padding that works would likely be a "magic number" that is not particularly flexible.

Doing nothing is another possibility. It's not obvious how widespread this issue is but @chloekraw proposed it in the linked ticket above so it has come up. It also doesn't make much sense to add the glyph metrics but not expose ways to make them useful for end users.

## Design

A `text-vertical-align` property would be clear and produce consistent results. It's easier to use than padding. The major downside is a new style spec property but this property would add a considerable amount of flexibility for map designers while solving an existing bug.

The property would be an enum with possible values of "top", "middle", "bottom", "baseline" and possibly "super" and "sub" (I'm not particularly familiar with what glyph metrics would be returned by node-fontnik so perhaps some of these options would be more difficult to implement than others).

### Mock-Up

```
layout: {
"text-field": ["format",
"foo", { "font-scale": 1.2 },
"bar", { "font-scale": 0.5, "text-vertical-align": "top" }]
```

### Concepts

It would be similar in concept to the existing CSS property `vertical-align`. The concept of working with text baselines should be familiar to many front end developers and designers.

### Implementation

Depends on #191 for glyph metrics

The implementation would be a symbol layout property that takes an enum value.

Start by reading prerequisite issue #191 and the symbol layout property described here, then confirm what glyph metrics are available. The work is done when the supported vertical-alignment values and behavior are settled and the new style property is implemented using those metrics.

Written by the indexing model from the issue text.