Add `driver-type`

[dgr]

Problem/Opportunity
Currently, Etaoin has no public way to determine the type of driver that is running. There is a driver? API, but this tests whether the driver is of a particular type (e.g., answers the question, "Is this driver :chrome?"). Consequently, various pieces of Etaoin code have been using dispatch-driver, but this is not a public API and has an odd name that suggests its internal function but is not suited for external users.

Proposed Solution
Create a public function named driver-type that returns one of :chrome, :firefox, :safari, or :edge. This public function merely calls dispatch-driver under the hood.

Alternative Solutions
Muddle through with the current status quo.

Additional context
None.

Action
I'll submit a PR.