Add iterate functions to collection types in vscode.d.ts

[mjbvz]

vscode.d.ts defines a few collection types: DiagnosticCollection, DataTransfer, EnvironmentVariableCollection, TestItemCollection. Currently we only expose a forEach method on each of these to iterate through them. This works but is annoying for a few reasons:

1. You can't use the with for...of loops

2. forEach doesn't play nicely with async/await. This is specifically an issue for DataTransfer where you end up writing code such as:

const promises: Thenable<string>[] = [];
dataTransfer.forEach(x => {
	promises.push(x.asString());
});
const result = Promise.all(promises);

3. forEach uses a closure which messes up TS's type guards:

let thing: string | number = 1;
if (typeof thing === 'number') {
	return;
}
dataTransfer.forEach(x => {
	thing.charAt(0) // 💥  Thing is back to being a `string | number` again
});

Potential fixes

Two potential approaches:

1. Add .entries() function to these collections. These would behave like new Map().entries()

2. Make these types iterable directly by defining a [Symbol.iterabor] function on them