Non-breaking

• Merge/close all pull requests.
• Move .js files to src/, tsc, add { "exports": { "./": "./lib/" } } to package.json.
• Add stub files to avoid breaking for Node <12 Drop Node <12 compat
• Change extension to .ts and add type annotations from .d.ts files.
  • router.ts
  • telegram.ts
  • context.ts
  • composer.ts
  • core/network/client.ts
  • telegraf.ts
• Configure formatting/linting of .ts files.
• Move the main documentation to typedoc (Move to typedoc #1205)
• Update examples (Typecheck examples #1177, Types and examples for scenes and wizards #1194)

Help wanted

• scenes/
• stage.ts
• extra.ts (Extra::caption ovewrties itself #1076)
• session.ts (ContextMessageUpdate type definition missing 'session' #914)
• Complete type annotations, noImplicitAny.
• Fix eslint warnings and https://github.com/telegraf/telegraf/search?q=ts-expect-error.

Breaking

• index.ts (awaiting esModuleInterop doesn't affect reexports microsoft/TypeScript#39464)
• mjs-entry
• "exports": { "import": "./lib/index.mjs", "require": "./lib/index.js" }
• markup.ts (rewrite)
• Start building .d.ts files from source.
• Resolve related issues.
• (optional) denoify.