Support ES2021 Logical Assignment

[ljqx]

This PR fixes #2057.

• scanner.ts is updated to accept &&=, ||=, ??=.
• parser.ts is updated to treat &&=, ||=, ??= as assignment operator.
• Unit tests and test-262 whitelist are updated.

[lahma]

@ljqx can you please rebase this against master to make sure the optional attribute is not missing in the JSON tree presentation?

[lahma]

@ariya I've tested this on .NET side and successfully integrated to Jint codebase, passing all tests related to logical assignment.