Add tests for LineContinuation in StringLiteral

Adding prefixed string to compare with BigInt

Formalize asynchronous failure conditions

Previously, the error message generated by failed asynchronous tests was
generic and underspecified. Improve the format and explicitly document
it in project's interpreting guidelines.

There's a lot of work like this to do, but not enough time in the day to reach it all.

Improve and extend the tests for ApplyOptionsToTag.

Add some more Locale tests.

These tests are againt a proposed fix for
https://github.com/tc39/ecma262/issues/753

They seem to pass in V8, JSC and SpiderMonkey, though ChakraCore
hews slightly closer to the previous specification.

- fix mixed up test & info
- add Symbol feature
- maximize length is 0
- maximize configurability is true

Update CONTRIBUTING.md

Add test for numeric literal followed by identifier

typo in constructor Test252Error

Added tests for relational comparison among BigInt and Symbol

Tests for proposal-json-superset

Added tests of relational comparison among BigInt and boolean

Some of these are based on work by André Bargull in
<https://bugzilla.mozilla.org/show_bug.cgi?id=1433303>.

Various test cases for cross-browser compliance bugs

Defines $262.agent.monotonicNow()