Migrate to TypeScript

[Hellzed]

This is a 1:1 migration of sepa.js to TypeScript (from plain Javascript).
Rollup is introduced as the required bundler.

DONE:

• basic conversion to TypeScript syntax (to review, I recommend diffing the original 'lib/sepa.js' and the new 'lib/sepa.ts'),
• as a nice extra, it actually exports Sepa.Document, Sepa.PaymentInformation and Sepa.Transaction, so if you don't want to use the original procedural API to build the SEPA document, you don't have to.

TODO:

• split the single file into smaller, more manageable modules,
• export consistent interfaces/types,
• use setters to validate data as it's added,
• add more type hints,
• improve the build step (build bothe CJS and ESM modules, add a minified version with a sourcemap?).

KNOWN ISSUES:

• works in Node, but probably not in browsers (not tested), so it's not equivalent (yet) to the publicly available lib sepa.js.

When outstanding issues are fixed, let's publish this to our private NPM repo.
@moutard The SEPA abstraction interfaces defined in Spendesk server are even better, but I'm not sure yet how to add them on top of this).