Implement #elif preprocessor directive

[Thorium]

Implement #elif preprocessor directive

Fix for fsharp/fslang-suggestions#1370

Checklist

• Test cases added
• Performance benchmarks added in case of performance changes
• Release notes entry updated:

  Please make sure to add an entry with short succinct description of the change as well as link to this pull request to the respective release notes file, if applicable.

Edit: RFC

[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/11.0.0.md

[Martin521]

Nice work.
But this is not a fix, but a new feature that needs an RFC.
(Which should also document the effect on tooling (where it is / can be a breaking change).

[T-Gro]

Thank you for the contribution.
The RFC makes sense, it also acts as a notification mechanism for other interested parties (IDEs and Fantomas come to mind).
Parts of the RFC text on usage should then also make their way into the docs.

It's definitely good to see this early in the F#11 development cycle, as it gives enough time to others for adjustment 👍

[Thorium]

How is the RFC process working? Do I have to make it, or someone else?

[auduchinok]

Could you please also add lexing and parsing tests for the compiler service? TokenizerTests or SyntaxTreeTests would be a good place to start.

[Martin521]

How is the RFC process working? Do I have to make it, or someone else?

It is typically created by somebody interested in the feature. The process is described in the readme of the lang-design repo and further documentation is in the RFC template. I think the RFC can be rather brief in this case.

[T-Gro]

The RFC is now approved and merged.

[T-Gro]

@Thorium : Would you mind if I pick some of your design and try finishing this in a fresh PR?

[Thorium]

You are welcome