Add js api

[trungleduc]

Code Changes:

• New token IForkManagerToken in @jupyter/docprovider-extension.
• This token implements the following interface:

    /**
     * Creates a new fork for a given document.
     *
     * @param options.rootId - The ID of the root document to fork.
     * @param options.synchronize - A flag indicating whether the fork should be kept
     * synchronized with the root document.
     * @param options.title - An optional label for the fork.
     * @param options.description - An optional description for the fork.
     *
     * @returns A promise that resolves to an `IForkCreationResponse` if the fork
     * is created successfully, or `undefined` if the creation fails.
     */
    createFork(options: {
        rootId: string;
        synchronize: boolean;
        title?: string;
        description?: string;
    }): Promise<IForkCreationResponse | undefined>;
    /**
     * Retrieves all forks associated with a specific document.
     *
     * @param documentId - The ID of the document for which forks are to be retrieved.
     *
     * @returns A promise that resolves to an `IAllForksResponse` containing information about all forks.
     */
    getAllForks(documentId: string): Promise<IAllForksResponse>;
    /**
     * Deletes a specified fork and optionally merges its changes.
     *
     * @param options - Options for deleting the fork.
     * @param options.forkId - The ID of the fork to be deleted.
     * @param options.merge - A flag indicating whether changes from the fork should be merged back into the root document.
     *
     * @returns A promise that resolves when the fork is successfully deleted.
     */
    deleteFork(options: {
        forkId: string;
        merge: boolean;
    }): Promise<void>;
    /**
     * Signal emitted when a new fork is added.
     *
     * @event forkAdded
     * @type ISignal<IForkManager, IForkChangedEvent>
     */
    forkAdded: ISignal<IForkManager, IForkChangedEvent>;
    /**
     * Signal emitted when a fork is deleted.
     *
     * @event forkDeleted
     * @type ISignal<IForkManager, IForkChangedEvent>
     */
    forkDeleted: ISignal<IForkManager, IForkChangedEvent>;

[github-actions]

👈 Launch a Binder on branch trungleduc/jupyter-collaboration/fork-js-api

[davidbrochart]

Thanks so much @trungleduc, this is great.
I just have a minor comment.