fix(isJWT): add compatibility for JWE (RFC7516)

[damikael]

JWT (RFC7519) can be:

• JWS (header.payload.signature) - RFC7515
• JWE (header.encryption_key.initialization_vector.ciphertext.authentication_tag) - RFC7516

This PR add compatibility to validate a JWT as JWE if the second parameter is true.

Checklist

• [x] PR contains only changes related
• [x] README updated (where applicable)
• [x] Tests written (where applicable)

[codecov[bot]]

Codecov Report

Merging #2031 (3243b61) into master (86a07ba) will not change coverage. The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master     #2031   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          104       104           
  Lines         2203      2206    +3     
  Branches       477       480    +3     
=========================================
+ Hits          2203      2206    +3     

Impacted Files	Coverage Δ
src/lib/isJWT.js	100.00% <100.00%> (ø)

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