DSL file: add a mandatory key for the "type" of DSL?

[presentation.pdf](https://github.com/eclipse/gemoc-studio/files/2105635/presentation.pdf)

(just opening this issue as a reminder with the slides I has for EclipseCon, I'll detail the description when I have time)

Start by reviewing presentation.pdf, the only artifact linked in the issue, to understand the proposed DSL type key. The issue does not name source files, tests, an entry point, or acceptance criteria; clarify the intended syntax, affected DSL tooling, and what constitutes completion before implementation.

Written by the indexing model from the issue text.