Mapbox-gl-js: Rename `layer` option to `layers` for featuresIn/At requests

[tristen]

I've seen this come up a couple times where someone has typed layers instead of layer in the property options for featuresAt or featuresIn requests. If the typo is not caught the effect is elusive (_appears_ to sort of work as the layer you want is captured in the request but buggy as it captures others)

Propose:

• layer continues to accept a single string value or an array of strings
• layers accepts an array of strings

[lucaswoj]

someone

:+1: I've done this many times.

[tmcw]

:-1: rename to layers, require an array of strings as an argument. One true way.

[averas]

I agree with @tmcw.

[peterqliu]

Agree with one true way, though should we continue to support the singular form, to avoid a breaking change?

[jfirebaugh]

Let's support an array-valued layers property only, as part of #2052 / #2224, which are already breaking changes.

cc @ansis