Implement transform for nullish-coalescing operator (#6483)

* Implement transform for nullish-coalescing operator

* Update example output

* Switch from BinaryExpression to LogicalExpression

* Address review comments

- Use generateUidIdentifierBasedOnNode
- Inline "??"
- Clone ref node
- Move "??" to LogicalExpression in babel-types

* Fix reference to @babel/helper-plugin-test-runner

* Fix reference to @babel/plugin-syntax-nullish-coalescing-operator

* Don't use parent scope

* Remove .vscode from .gitignore, change 'lib/index.js' to 'lib'

* Ensure `document.all ?? 0 === document.all`

* Fix note and copy to an inline comment

packages/babel-plugin-syntax-nullish-coalescing-operator/package.json

@@ -0,0 +1,13 @@
+{
+  "name": "@babel/plugin-syntax-nullish-coalescing-operator",
+  "version": "7.0.0-beta.3",
+  "description": "Allow parsing of the nullish-coalescing operator",
+  "repository": "https://github.com/babel/babel/tree/master/packages/babel-plugin-syntax-nullish-coalescing-operator",
+  "license": "MIT",
+  "main": "lib",
+  "keywords": [
+    "babel-plugin"
+  ],
+  "dependencies": {},
+  "devDependencies": {}
+}