feat(core): relax reserved property names

[mttrbrts]

Partially addresses Issue #572

This PR relaxes the parsing and core API restrictions on identifier names. The following names are now permitted:

• false
• true
• null

Changes

• Adds explicit test cases which give good coverage of valid & invalid identifiers
• Changes Parser rules to allow NullLiteral and BooleanLiterals as identifiers
• Removes the explicit reserved keyword check in the core API. This was only introduced recently to address a ReDOS limitation when generating sample JSON files regexes (negative lookahead was not supported). BREAKING CHANGE(core): limit regex to ReDoS safe patterns #484
• Upgrades Unicode character categories to Unicode 15.0.0

Flags

• I believe that this should be considered a non-breaking change, as it loosens the constraints on identifiers.

Author Checklist

• Ensure you provide a DCO sign-off for your commits using the --signoff option of git commit.
• Vital features and changes captured in unit and/or integration tests
• Commits messages follow AP format
• Extend the documentation, if necessary
• Merging to main from fork:branchname

[mttrbrts]

@jonathan-casey

[dselman]

I assume imposing the identifier constraints on the AST will be in a follow-up PR?

[mttrbrts]

I assume imposing the identifier constraints on the AST will be in a follow-up PR?

Yes, that's right