support spec store

[unional]

Support a spec store so that within one spec, the test has an option to store repeated calls.

The main usage of this store is for Zucchini,
where the step can store cached data without relaying on module scope,
which does not work when there are multiple tests from the same test suite.

[unional]

i.e., something like:

defineStep('...', ({ store } , ...) => {
  store.get(...)
  store.set(...)
})

The store will persist within one test, or one test suite. Will need to see what are the use case of each and then see how to support them