This Pull Request refactors the code to TypeScript.
This has the benefit of not having separate typings which are maybe wrong. This should also improve code readability and maintainability.

While this should be a simple refactoring in general I decided to introduce two changes to simplify code and have more strict typings, one change is breaking.

The first change is the internal structure of the locale repositories. Before this change the Repository is structured with the object tree structure. When the resourceKey a.b.c in language en is requested the following value is read: {en: {a: {b: {c: value}}}}.
With this change the dot notation is already applied when loading into the repository and not every time a key is used. This results in a simpler structure of the repository: {en: {'a.b.c': value}}. Having that simplifies internal typings (and has an insignificant performance benefit). This change is not breaking and shouldn't be noticed by users.

The breaking change is the way to import/require and use this library.
Before this you could simply use any name you liked and use that:

const whatever = require('telegraf-i18n')
const i18n = new whatever(…)

With these changes you always have to use it like this:

const {I18n} = require('telegraf-i18n') // CommonJS
import {I18n} from 'telegraf-i18n'      // ES6 modules

const i18n = new I18n(…)

Allowing for the default import is not as easy with ES6 and CommonJS style imports and having the I18n class and functions like reply. After some testing it wasnt worth my time: There is a way to use it and its not hard to adapt to that.
If someone has some suggestions how to solve that elegantly: I'm curious :)

As NodeJS 8 is not supported anymore and telegraf now requiring NodeJS 12 this is also another breaking change. But as you need NodeJS 12 or higher for telegraf anyway users probably wont notice.

fixes #30
fixes #33
fixes #35
fixes #36
fixes #37
fixes #38

Feel free to comment on the changes. I'm sure someone can spot something to improve or has better ideas how to do something.

I tested the changes in multiple of my TypeScript Bots. I do not have any running JavaScript bot left so it might be nice if someone else is willing to test on that. Especially If the import works well.