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

👍 I've done this many times.

[tmcw]

👎 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