feat: add Composer::boundary()

[KnorpelSenf]

As suggested by @rayz1065

[rayz1065]

Suppose I want to write a tree surrounding plugin, with regular middleware plugins I would simply be able to write:

type Options = { foo: string };
function myPlugin<C extends Context>(options?: Options): Middleware<C> {
    return (ctx, next) => {
        // ...
    };
}
bot.use(myPlugin({ foo: "bar" }));

It would be nice to be able to do something similar for a surrounding middleware:

export type SurroundingMiddlewareFn<C extends Context = Context> = (
    ctx: C,
    tree: () => Promise<{ nextCalled: boolean }>,
    next: NextFunction,
) => MaybePromise<unknown>;

function mySurroundingPlugin<C extends Context>(
    options?: Options,
): SurroundingMiddlewareFn<C> {
    return (ctx, tree, next) => {
        // ...
    };
}
const surrounded = bot.surround(mySurroundingPlugin({ foo: "baz" }));