v0.2.1 released

[anakrish]

https://github.com/microsoft/regorus/releases/tag/regorus-v0.2.1

Key Highlights

• AST is now available in JSON format. JSON format allows exposing the AST while keeping the implementation private.
• The list of policy files and contents can be obtained from engine. This enables auditing in certain workflows.
• The lexer has been made flexible to support other policy languages.