Small improvements in the Credential interfaces, removal of exposure of internal structures in the PEX class. Small bugfixes

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Breaking change: class and package renamed in v0.6.0!

As part of introducing Presentation Exchange v1 and v2 feature based detection support to our Presentation Exchange
library and not reaching version 1.X yet, we decided to change the name of both the package and the main entry class:

• The package was changed from @sphereon/pe-js to @sphereon/pex
• The main class was changed from PEJS to PEX. The latter class has internal feature detection support on the
  provided definition, delegating the actual implementation to the new PEXv1 or PEXv2 class internally. If you don't
  want the automatic feature detection you can also choose to use the PEXv1 and PEXv2 classes directly.

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• Changed:

  • Next to @context evaluation for V1 schema values, we now look at the credentialSchema
  • Seperated Credentials and VerifiableCredential public interfaces into JSON-LD and JWT versions

• Fixed:

  • We were exposing some internal structures in the PEX class
  • Group validation assumed an array being present, which could be a string