Use tokenParsed instead of jwt-decode

[tswistak]

Hi!

While browsing libraries code, I've found that it's using jwt-decode to decode the JWT token fetched from the Keycloak server. I think it would be better to use the tokenParsed property that is available in Keycloak's instance. There is all the data from the JWT token, and it won't be needed to enforce users to install jwt-decode.

Cheers!

[Jogai]

This fork does that: JoseGoncalves/vue-keycloak