Support Slack auth.revoke

• Add auth.revoke, which revokes the calling token, from the Authorization header or the token POST field, so later Web API calls return invalid_auth, with test=true checking without revoking.
• Keep token_revoked for apps removed by apps.uninstall, including in auth.revoke itself, with regression coverage, the coverage matrix entry, and docs.

References:

• auth.revoke response { "ok": true, "revoked": true }, the test argument, the token in the header or as a POST parameter: https://docs.slack.dev/reference/methods/auth.revoke
• invalid_auth "Method was called with invalid credentials", and token_revoked for a deleted user or workspace or a removed app: https://docs.slack.dev/reference/methods/auth.test

Covers the auth.revoke part of #327.