Add version of intl402/6.2.3.js which tests via Intl.getCanonicalLocales

Add messages to asserts and use mixed quotes instead of escapes

Fix CONTRIBUTING.md description of the info frontmatter tag

* Add missing generators feature flags

* Generate files

* Add generators flags

* fixup! Add generators flags

Update async iteration tests with respect to pending spec change

Update behaviour based on changes from https://github.com/tc39/ecma262/pull/988.
The actual spec change PR for async iteration is not yet uploaded.

This does not include any changes to Async-from-Sync Iterator.

* additional BigInt JSON tests

* single quotes

* BigInt stringify order of steps

* Improve tests for BigInt.prototype.valueOf

* fixup! Improve tests for BigInt.prototype.valueOf

* fixup! Improve tests for BigInt.prototype.valueOf

* fixup! Improve tests for BigInt.prototype.valueOf

typeCoercion.js is included in several tests which are not marked as `features: [BigInt]`.
Since BigInt is new feature, we should not make the above unrelated tests failed due to
lack of BigInt syntax support.

Close #1252.

* BigInt valueOf tests

* add features from typeCoercion.js

As per https://github.com/tc39/ecma402/pull/160 the formatToParts.length is set to 1.

Changes to the instructions for interpreting tests will likely produce
new failures for consumers who are updating between revisions of
Test262. Introduce a machine-readable convention for signaling
substantive changes.

https://github.com/tc39/ecma262/pull/988 changes the iteration protocol
such that the "next" method is only loaded from the iterator object once
during the prologue of iteration, rather than during each step.

BigInt.asUintN tests

Change destructuring tests to not rely on exact evaluation order for property references

Fixes #1202