Multiple Label Styles with Format Expression

[riastrad]

Given that mapbox/mapbox-gl-native#12985 has landed, it's now possible to style a SymbolLayer's text field with multiple fonts, sizes, etc. It would be great to have an example in the app that showcased this (with a bit more polish than mapbox/mapbox-gl-native#12985 (comment)). Something as simple as the example that was included in the gl-native test app should do just fine.

Note that this is a slightly different example than #982 as it will have multiple styles within the textField. Though this would be complimentary to that, it shows off a different possibility for the expression.

cc: @langsmith @chloekraw

[langsmith]

#984 is related to this ticket 👥