add `spec-assertion-thrown?` test declaration

[thumbnail]

Problem statement

spec-assertion-thrown? is a assert-expr symbol, not a function. So it misses highlighting and a proper refer.

This has a couple of caveats:

• editors can't show arglist
• lack of traceability. The assert-expr is registered when speced.def is loaded, which might be in a different ns
• formatters might remove [nedap.speced.def] as a dependency, if it is only loaded for registering the spec-assertion-thrown?-matcher.

Proposal

add test-ns with a declaration for the assert-expression. Similar to matcher-combinators:
https://github.com/nubank/matcher-combinators/blob/b3da18923f0aa6a5ef924e9303f690a3efa1d978/src/cljc/matcher_combinators/test.cljc#L10-L11

[vemv]

SGTM, it's not perfect but I see the benefits. One could even go furher and add doc metadata to the declare which would be another benefit.