Add release notes for --typecheck-only flag support in F# Interactive

Co-authored-by: T-Gro <[email protected]>

Author: Copilot

docs/release-notes/.FSharp.Compiler.Service/10.0.100.md

@@ -1,6 +1,7 @@
 ### Added
 * Add opt-in warning attribute not valid for union case with fields [PR #18532](https://github.com/dotnet/fsharp/pull/18532))
 * Add support for `when 'T : Enum` library-only static optimization constraint. ([PR #18546](https://github.com/dotnet/fsharp/pull/18546))
+* Add `--typecheck-only` flag support for F# Interactive (FSI) scripts to type-check without execution. ([Issue #18686](https://github.com/dotnet/fsharp/issues/18686))
 
 ### Fixed