Formalize our approach to license headers in rascal (and write down why BSD2 vs BSD3)

[DavyLandman]

@PaulKlint in #488

The situation around licensing has evolved and there is now another option: we could use SPDX license identifiers from https://spdx.org/licenses/. They are widely supported and also GitHub recognizes them.

In our case the license would reduce to something like this:

@license{Copyright (c) 2018-2025, NWO-I CWI, Swat.engineering and Paul Klint; SPDX-License-Identifier: BSD-2-Clause}

[We could even decide to use BSD-3-Clause, but that is another debate]

I've moved the discussion to this repo, as this is where we normally discuss these larger topics.