Misc fixes to @babel/types AST definitions

[JLHwung]

Q	A
Fixed Issues?	See below
Patch: Bug Fix?
Major: Breaking Change?
Minor: New Feature?
Tests Added + Pass?	Yes
Documentation PR Link
Any Dependency Changes?
License	MIT

• Allow BigIntLiteral as property/method key
• Refine error message when parsing decorator codes without the corresponding plugin: Now Babel will throw "Support for the experimental syntax 'decorators' isn't currently enabled" instead of referring to the legacy decorators plugin.
• Validate ClassPrivateProperty.static (boolean defaults to false)
  Technically this one may be a breaking change, because now Babel types will throw when the provided static is non-boolean, though I doubt anyone has ever spotted it.
• Disallow "constructor" as ClassPrivateMethod.kind

[babel-bot]

Build successful! You can test your changes in the REPL here: https://babeljs.io/repl/build/52440/

[JLHwung]

BTW TIL we have "decorators"

babel/packages/babel-types/src/definitions/core.ts

Lines 492 to 497 in b67e369

	decorators: {
	validate: chain(
	assertValueType("array"),
	assertEach(assertNodeType("Decorator")),
	),
	},

for the PatternLike nodes such as Identifier/ArrayPattern/ObjectPattern. Does @f x has ever been supported in legacy decorators proposal? Currently Babel throws "leading decorators must be attached to a class declaration" anyway. If this property is redundant we can consider remove it from Babel 8.

[nicolo-ribaudo]

@JLHwung That's probably for TS parameter decorators:

class A {
  f(@d a, @d { x, y }) {}
}

Changing it goes in the same direction as #9545.