refactor: drop oauth_ (#637)

change `oauth_pkce` to be just `pkce` so that magic link, recovery, etc
can make use of the same route

Upd: realized that there's still a need to encode what kind of flow
resulted in the token in order for `issueRefreshToken` to track where
the token came from

src/GoTrueClient.ts

@@ -401,7 +401,7 @@ export default class GoTrueClient {
     const { data, error } = await _request(
       this.fetch,
       'POST',
-      `${this.url}/token?grant_type=oauth_pkce`,
+      `${this.url}/token?grant_type=pkce`,
       {
         headers: this.headers,
         body: {