Error on invalid declarations in type definitions.

[edgarfgp]

Description

Fixes #10066

Checklist

• Test cases added
• Release notes entry updated

[github-actions]

❗ Release notes required

---

✅ Found changes and release notes in following paths:

Change path	Release notes path	Description
src/Compiler	docs/release-notes/.FSharp.Compiler.Service/10.0.100.md
LanguageFeatures.fsi	docs/release-notes/.Language/preview.md

[auduchinok]

I strongly believe it should be an error, just like in #15334. If we're targeting a new language version, I propose we make it an error.

[edgarfgp]

I strongly believe it should be an error, just like in #15334. If we're targeting a new language version, I propose we make it an error.

Im kind of in the same boat. Happy to change it. Just need @T-Gro blessing : )

[T-Gro]

Approved also with changing it from warn to error for users of languageversion=10.

[psfinaki]

Great job, as always Edgar :)

[edgarfgp]

@T-Gro Any chance I get some help updating the last failing .bsl tests. Im on a macOS and for some reason the use usual export TEST_UPDATE_BSL=1 & dotnet test does not work. Thanks