Make multi input a form control

[camertron]

Description

The multi input in the forms framework is designed to wrap multiple inputs with the same name, only one of which may be visible at once. Why would anyone need such a thing, you ask? Imagine a form with a country dropdown. When the user selects "United States," the region input should show a list of US states. When the user selects "Canada," the region input should show a list of Canadian provinces. If any other country is selected, region becomes a free-form text field.

Currently, the multi input does not wrap its constituent inputs in FormControl - the inputs are expected to already be wrapped in FormControl. However I believe the label and validation message, i.e. the additional markup added by FormControl, should appear in the DOM only once. They should wrap the three inputs (US states, Canadian provinces, free-form text). Javascript handles showing and hiding the actual <select> and <input> elements.

To that end, this PR accomplishes the following:

1. Adds a form_control boolean attribute to all inputs that controls whether or not FormControl should wrap the input. If form_control: false is passed, only the input itself is rendered.
2. Modifies the multi input to wrap all its children in a single FormControl, and passes form_control: false when creating them.
3. Adds the primer-multi-input custom element that provides an activateField method for switching between constituent fields.

Integration

Does this change require any updates to code in production?

No. Nothing is using this field in production.

Merge checklist

• Added/updated tests
• Added/updated documentation
• Added/updated previews

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🦋 Changeset detected

Latest commit: 5a4a3f6

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@primer/view-components	Patch

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