Fix flag v bugs in custom regex engine

[adams85]

Summary

This PR makes some minor corrections to the custom regexp engine failing to execute some valid regexp patterns.

Details

The custom engine incorrectly rejects some regexp constructs in flag v patterns:

• ClassSetSyntaxCharacter outside class sets, e.g. /-/v,
• ClassSetReservedDoublePunctuator outside class sets, e.g. /&&/v
• escaped ClassSetReservedPunctuator inside class sets, e.g. /[\!]/v

Linked issue

n/a

Test plan

• Added or updated unit tests in Jint.Tests
• Ran dotnet test --configuration Release locally
• For ECMAScript spec changes: ran Jint.Tests.Test262 and confirmed no regressions
• For interop changes: covered in Jint.Tests/Runtime/Interop
• For perf changes: included before/after numbers from Jint.Benchmark

Breaking change?

No

[lahma]

Thank you as always!

[adams85]

My pleasure :)

BTW, working on another one, so more to come soon.