Fix parameter expression get binding

[JLHwung]

Q	A
Fixed Issues?	getBinding will return declaration inside the function body for patterns in function parameters.
Patch: Bug Fix?	Yes
Tests Added + Pass?	Yes
License	MIT

This PR is a rework of #10055. In this PR I extend the definition of Scope to include all the Pattern nodes when it immediately descends from the params of a Function node. By doing so we can intercept the ancestry queries in scope.getBinding and skip the binding if it is not a param-kind, because the spec (9.2.10, Step 28) requires that any closure defined in the parameter expressions should not access the declarations inside the function body.

An example can be found in the scope unit test:

babel/packages/babel-traverse/test/scope.js

Lines 77 to 92 in fb4125a

	it("should not has visibility of declarations inside function body", () => {
	expect(
	getPath(
	`var a = "outside"; (function foo(b = a) { let a = "inside" })`,
	)
	.get("body.1.expression.params.0")
	.scope.getBinding("a").path.node.init.value,
	).toBe("outside");
	});
	it("should has visibility on paramater bindings", () => {
	expect(
	getPath(`var a = "outside"; (function foo(b = a, a = "inside") {})`)
	.get("body.1.expression.params.0")
	.scope.getBinding("a").path.node.right.value,
	).toBe("inside");
	});

Personal sidenote:
This PR is dedicated to @nicolo-ribaudo, who gave insightful comments to #10055, one of my first works when I spent days to merely learn what is scope, and who inspires me and encourages me to work on Babel. Thank you and Merry Christmas @nicolo-ribaudo!

[JLHwung]

If a Pattern is a child of Function, it must be one of the params key because both body and typeAnnotation can not be a Pattern.

[nicolo-ribaudo]

If you need to add a comment to explain your PR, then why don't add it directly in the code? 😉

[JLHwung]

This case is one (likely the only one?) of the exception that although the Pattern is descended from Function, the scope of Pattern is not descended from the one of Function. This breaks the assumption that a scope is always initialized from the scope of its nearest Scopable ancestry, which becomes scope.parent later.

babel/packages/babel-traverse/src/path/context.js

Line 113 in f651f22

export function setScope() {

I feel like it is more cumbersome to fix that on scope.parent accessor, so I implement the fix on getBinding.

[nicolo-ribaudo]

Thanks! This PR is definitely better than the other. I'm impressed by how much you have learned in the last six months.

[nicolo-ribaudo]

If you need to add a comment to explain your PR, then why don't add it directly in the code? 😉

[nicolo-ribaudo]

@JLHwung wrong button?

[JLHwung]

@nicolo-ribaudo Oops.