misc(axe): rename axe types

[paulirish]

small tweak.

we use axeResult in two places for two different things. I've renamed both to differentiate more clearly.

1. in the gatherer to refer to the return value of axe.run().
   • axe-core calls this the "results object", so i've added an s to here to align.
2. the artifacts.d.ts type that refers to an individual axe rule's results.
   • I've renamed it to AxeRuleResult to clarify it's just a single axe rule being described. axe-core uses this term, too.

[adamraine]

LGTM