Add a generic, "core/forms" store for forms

Issue #1211 required some forms that didn’t warrant creating a bunch of actions, selectors, etc. for their data, but we shouldn’t be using useState to manage form data because it requires passing around state from components and negates a lot of the wins we get from using a Redux-backed datastore.

We talked about this on a call today, and we should have a generic, 'core/forms' data store with a very basic API for getting/setting state across forms in the app.

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Acceptance criteria

• There should be a new file, assets/js/googlesitekit/datastore/forms, that registers a new form data store, under 'core/forms'.
• It should provide a single selector: getValue( formName, key )
• It should provide a single action: setValues( formName, { key: value } ) where the second argument is a map of keys to set and their respective values.

Implementation Brief

• Create a new datastore in assets/js/googlesitekit/datastore/forms that registers a new form data store, under 'core/forms'.
• Create a assets/js/googlesitekit-forms.js that registers the core/forms store on the default registry.
• Add a googlesitekit-forms asset in Assets.php, similar to googlesitekit-api, googlesitekit-data, etc. that would load the core/form datastore into the default registry.
• Add a selector: getValue( formName, key ). This should return the value stored in the state[ formName ][ key]; undefined if the formName or key doesn’t exist.
• Add an action: setValues( formName, { key: value } ) where the second argument is a map of keys to set and their respective values. If the specified formName doesn’t exist; it should be created.

QA Brief

• Since this is an engineering-only feature for now, specific QA isn’t really needed.

Changelog entry

• Introduce core/forms datastore to manage form state.

[tofumatt]

Sounds good; I’ve updated the IB with your suggestions, with one exception: I don’t think the extra setValue function provides any extra value. The functionality from setValue( 'foo', 'bar' ) is already present in setValues({ foo: 'bar' }), but the latter is more extensible if one wants to add more items.

I don’t think there’s much benefit in having the single-variable one, especially if we’re the only ones using this API for now. But it’s an extra API to maintain without much benefit, so I’d prefer not to add it. (It is easy, but it’s also not super-useful in my opinion.)

[ryanwelcher]

Looks good to me! QA ✅