MJML6 Migrate from Lerna to NX, and introducing Typescript

[iRyusa]

There's no ETA on this as I work on this on my free time.

I don't know if anyone will read this but given the time I have it's hard to find time to add new features into the codebase our test coverage purely sucks, releasing new version sucks, installing the project locally sucks.

So what's the plan ?

• [ ] Migrate from outdated Lerna to Nx
• [ ] Remove Babel.
• [ ] Add a very loose Typescript config and start typing as much public API as possible
• [ ] Ship d.ts file
• [ ] Make automatic release from git tags on NPM

This would at least make MJML contribution WAY EASIER for everyone wanting to get involved in the project.

Why a v6 when v5 is not out yet ?

Ok so v5 was behind an "experimental" branch because of changing "core" elements for minifying/beautify the HTML, numbers might not be relevant, but there's 600k+ download per week of MJMl 4. If v5 was released, that would mean a lot of you would just jump in v5 finding new bug and giving my limited time I could work on MJML5 I wouldn't be able to patch it right away. I don't feel safe releasing new version of MJML (so imagine for a major one when you replace core dep), so that's why v6 is needed to ensure we can provide safe & faster release.

New features when ?

Not planned for now, sorry for pushing 2 major versions without any groundbreaking feature.