Make nonempty array generic type, use to describe ResourceLoader.getEnvironments()

[jlrobins]

Make a type NonEmptyArray<T> = [T, ...T[]]; somewhere useful.

Use it to annotate the return type for ResourceLoader.getEnvironments()

Then remove any checks over return results for ResourceLoader.getEnvironments() having returned empty array, such as in GetEnvironmentsTool() check over if (!environments.length).

(CCloud accounts will always have at least one (possibly no resources) Environment).