fix: deep clone shared AST field 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

The shared AST field definitions could be mutated by

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

Lines 344 to 346 in 1c79224

	if (field.default !== undefined && builder.indexOf(key) === -1) {
	field.optional = true;
	}

This issue surfaced when I am reviewing https://github.com/babel/babel/pull/14730/files#diff-85c1fccaeecd3c19b928ca56b3730ba55d198ce4e61717f56fc6a6aa62a7f85fL1043, where the kind of ClassPrivateMethod changed from an optional property to a non-optional property (expected). Here we deep clone the shared definitions such that, for example, mutating TSPropertySignature's computed does not affect ClassMethod's.

[babel-bot]

Build successful! You can test your changes in the REPL here: https://babeljs.io/repl/build/52448/