Support forced evaluations like let!

[mjewell]

This library is analogous to let in rspec, but there isn't an equivalent for let!. We are using it for cypress tests and we almost always want to eagerly evaluate our given variables (because they correspond to API calls we want to resolve before the test starts), so it would be really nice if there was a shorthand for this eager evaluation. Currently our tests looks something like

given('a', () => createSomething());
given('b', () => createSomethingElse());

beforeEach(() => {
  given.a;
  given.b;
});

it('works', () => { /* */ });

But if this library supported that behavior we could remove the beforeEach. There are a few APIs I have thought might work but I'm open to other suggestions:

// lots of different naming options here - now / eager / bang / other ideas?
given.now('a', () => createSomething());

// this one is a bit more complex to implement but I think it can be made to work with template literal types
// https://www.typescriptlang.org/docs/handbook/2/template-literal-types.html
given('!a', () => => createSomething());
// or to make it look less like a boolean operator
given('a!', () => => createSomething());

I think this is relatively easy to implement by adding a __lazy__ property to the given object (pretty much identical to the __props__ property), and pushing in true or false based on how this function is called. Then you can add a beforeEach that checks the top of the list to decide if it should access given[key]. I'm happy to submit a PR if we can agree on an API.