[CS2] Destructuring (#4478)

* Output simple array destructuring assignments to ES2015

* Output simple object destructured assignments to ES2015

* Compile shorthand object properties to ES2015 shorthand properties

This dramatically improves the appearance of destructured imports.

* Compile default values in destructured assignment to ES2015

* Rename `wrapInBraces` to `wrapInParentheses`, and `compilePatternMatch` to `compileDestructuring`, for clarity; style improvements (no `==` or `!=`, etc.)

* Don’t confuse the syntax highlighter

* Comment Assign::compilePatternMatch a bit

* Assignment expressions in conditionals are a bad practice

* Optional check for existence that only checks `!== undefined`, not `!= null`, to follow ES convention that default values only apply when a variable is undefined, not falsy

* Add comments; remove unnecessary array splats in function tests

* The fallback destructuring code should apply default values only if `undefined`, not falsy, to follow ES spec

* Support destructuring in function parameters (first pass); catch destructured reserved words

* Destructured variables in function parameter lists shouldn’t be added to the function body with `var` declarations; treat splat array function parameters the legacy way to avoid rethinking #4005

* Remove redundancy in undefined-only check for existence; fix passing option to check

* Fix undefined redundancy

* Simplify getting the variable name

* Reimplement “check for existence if not undefined” without creating a new operator

* `Obj::isAssignable` should not mutate; pass `lhs` property in from `Assign` or `Code` to child arrays and objects so that those child nodes are set as allowable for destructuring

* Revert changes to tests

* Restore revised test for empty destructuring assignment