[6.x] Provide extra values through container and publish form

[jasonvarga]

This adds a provide prop to PublishContainer that'll allow extra values to be injected into a child component.

<Container
   :provide="{ foo: 'bar' }"
/>

import { injectPublishContext } from '@statamic/cms/ui';
const { foo } = injectPublishContext();
console.log(foo); // 'bar'

Also:

• Adds an isDirty computed to container, and provides it.
• Entry publish form provides isWorkingCopy and revisionsEnabled

Closes #13914