Fix #19457: lift CE constructs from plain let RHS in computation expressions

[T-Gro]

Summary

Fixes #19457

When a plain let p = rhs in body appears inside a computation expression, and
hs itself contains CE-only constructs (e.g. let! x = ...; x or do! ...; rest), the compiler incorrectly raised FS0750 ("This construct may only be used within computation expressions").

Fix

Rewrite the expression so those CE constructs are lifted into the enclosing CE, where they desugar correctly. This makes examples like:

\\sharp
task {
let result =
let! x = someAsync
x + 1
return result
}
\\

compile as expected.

Changes

• CheckComputationExpressions.fs: Added logic to detect and lift CE-only constructs from the RHS of plain let bindings inside CEs.
• ComputationExpressionTests.fs: Added tests covering nested let!/do! in plain let bindings within CEs.
• Release notes: Entry added to docs/release-notes/.FSharp.Compiler.Service/11.0.100.md.

Testing

• All new tests pass (ComponentTests Release: 13922 passed, 0 failed)
• SurfaceAreaTest passes (no public API change)
• Fantomas formatting check passes

[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/11.0.100.md