Export types CommandMiddleware, HearsMiddleware, etc

[IlyaSemenov]

With this PR, it's possible to decompose command handlers into separate functions/modules.

Example:

// bot.ts
import { Bot, Context } from "grammy"

import { commands } from "./commands"

export type BotContext = Context & {}

const bot = new Bot<BotContext>()
bot.use(commands)
bot.use(/* ... */)

// commands/index.ts
import { Composer } from "grammy"

import { BotContext } from "../bot"

import { start_command } from "./start"
import { other_command } from "./other"

export const commands = new Composer<BotContext>()

commands.command("start", start_command)
commands.command("other", other_command)
// ...possibly 20 more commands
// no need to include all implementations in a single file or use dummy composers

// commands/start.ts
import { CommandMiddleware } from "grammy"

import { BotContext } from "../bot"

export const start_command: CommandMiddleware<BotContext> = async (ctx) => {
  console.log(ctx.match) // properly typed
}

Closes #100.

[KnorpelSenf]

Thanks for working on #100! Would you mind exposing the other types in this PR, too?

[KnorpelSenf]

@IlyaSemenov could you have another look? I fixed it up.

[KnorpelSenf]

@all-contributors add @IlyaSemenov for code!

[allcontributors]

@KnorpelSenf

I've put up a pull request to add @IlyaSemenov! 🎉