Fix no-new-null for native private members

[Bharat Middha (bmiddha)]

Summary

• treat class members keyed by an ECMAScript PrivateIdentifier as private
• stop @rushstack/no-new-null from reporting null types that cannot escape through native private fields, methods, getters, or setters
• add regression coverage for each native private member form

Validation

• reproduced six false positives with the regression test before the fix
• rush build --to @rushstack/eslint-plugin --verbose
• rush test --only @rushstack/eslint-plugin --verbose (197 tests)

Dependents

#5939, #5941, #5942, and #5943 are stacked on this fix because their native-private migrations expose the false positive.