refactor: converting from monolith to monorepo, starting package decomposition

[emagnier]

It's been a while since we're talking about monorepos, so let's do it!

This PR is really just a first step. There's still quite a bit of work to modularize things further, but let's save that for future PRs.

Basically, in step 1, the romcal library is converted into a package within the monorepo @romcal/rite-roman1969. Later on, we can continue to refactor certain elements into different packages (for future PRs). I've already made a first one: @romcal/easter (which contains the code to calculate the date of Easter).

I think the key here is to stabilize the current configuration. I've continued to use npm to set up this monorepo. I'm more used to using pnpm than npm in monorepo projects, so I'd appreciate some help fine-tuning the configs and package.json with npm (for now, it's pretty basic).

Here's the start of the new structure:

packages/
    config/     # contains configuration files for current and future packages, defined once in this package
    easter/     # a first package, as an example
rites/
    roman1962/   # a new package, currently empty, waiting for future contributors ;-)
    roman1969/   # the 'romcal' library, moved here

For now, everything remains functional in the same way, using the npm package romcal (to avoid breaking changes).

If you'd like to contribute to this PR as well, feel free to do so 😉