Restore support for breaks along line labels

[mourner]

Restores support for line breaks on labels placed along lines, which was (possibly unintentionally) removed in #8706. It's a useful feature, and it brings GL JS on par with both GL Native, and the way map renders in Studio, because it loads RTL text plugin by default (so line breaks handling is delegated to the plugin which does support them).

Closes #7203, closes #9743, ref GLJS-228.

At the moment we can't test this with a render test, because all render tests run with the RTL Text plugin loaded globally:

mapbox-gl-js/test/integration/lib/render.js

Line 23 in ced4597

mapboxgl.setRTLTextPlugin('https://api.mapbox.com/mapbox-gl-js/plugins/mapbox-gl-rtl-text/v0.2.3/mapbox-gl-rtl-text.js');

We can try addressing this in a different PR. To prove that this PR fixes breaks along lines, I added a commit that disables RTL in tests to show how the multiline tests fail (disregarding RTL tests), then a commit return the fix in this PR with RTL still disabled to show that previously failing multiline tests pass, then finally restoring RTL to make the PR green. Example test:

before	after

Launch Checklist

• briefly describe the changes in this PR
• include before/after visuals or gifs if this PR includes visual changes
• write tests for all new functionality
• post benchmark scores
• manually test the debug page
• tagged @mapbox/map-design-team @mapbox/static-apis if this PR includes style spec API or visual changes — cc @tristen
• apply changelog label ('bug', 'feature', 'docs', etc) or use the label 'skip changelog'
• add an entry inside this element for inclusion in the mapbox-gl-js changelog: <changelog>Fix line breaks not working on labels along lines</changelog>