ES2020 BigInt

[lahma]

• can ignore .js file changes with filter - only updating test suite under BigInt folder
• new BigInt primitive type
• moved logic towards JsValue.Equals (===, strict) and JsValue.LooseEquals (==)
• about 800 new test cases passing

There might be things to improve, but I think some refactoring can be left later. I'm probably going to try separate operator overloading via decorators or something to make core logic more concise.

[lahma]

I think I still need to validate against latest test suite and bigint related tests.

[lahma]

All bigint related tests pass in my custom workbench after two latest commits, still requires new Esprima release for full compatibility.