Types: Improve DocumentActionComponent usage and extendability

[roch-numbered]

Is your feature request related to a problem? Please describe.
When developing a plugin, new actions aren't accepted by the typechecker.

my-plugin/plugin.tsx

import { MyPluginAction } from './actions/MyPluginAction';

export const myPlugin = definePlugin({
  name: 'my-plugin',
  document: {
    actions: (prev) => {
      // shortened version of the error
      // ❌ impossible to assign type 'string' to type '"delete" | "discardChanges" | "duplicate" | "restore" | "publish" | "unpublish" | undefined'
      prev.push(MyPluginAction);

      return prev;
    },
  }
});

my-plugin/actions/MyPluginAction.tsx

export const MyPluginAction: DocumentActionComponent = props => {};

MyPluginAction.action = 'myPlugin';

I can instead of DocumentActionComponent, simply use DocumentActionProps in the props object but I'll simply get a different error.

Describe the solution you'd like
Use keys of an interface instead of an union of string literals for DocumentActionComponent['action'].

packages/sanity/src/core/config/document/actions.ts

export interface DocumentActionKeys {
  delete: never
  discardChanges: never
  duplicate: never
  restore: never
  publish: never
  unpublish: never
}

export interface DocumentActionComponent extends ActionComponent<DocumentActionProps> {
  action?: keyof DocumentActionKeys
}

This will allows users to use declaration merging and add there own action names.

sanity.d.ts

import 'sanity'

// redeclare the sanity module
declare module 'sanity' {
  export interface DocumentActionKeys {
    myPlugin: never
  }
}

Describe alternatives you've considered
I've tried to use declaration merging to rewrite the type of DocumentActionComponent['action'] but that's not how TypeScript works.

Thx, cheers! ✌️