Fix tracer support for options (was crashing)

[mlaily]

Also see fable-compiler/Fable#4082

Fix options not being considered as DU in the tracer.

The previous implementation could actually crash:
if the input is Some(DUCase), Fable.Core.Reflection.isUnion returns true
because it does not "see" the option, and only see the wrapped DUCase.
Reflection.FSharpType.GetUnionCases() would then attempt to get the fields
of the input, and since option types are not considered DU under Fable,
it would throw.

Now there is a special case for options so they work the same as other DU for the tracer.

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Note: I also took the liberty to replace the build.fsx file with an equivalent build.fsproj project, because I couldn't manage to get it to build and run without errors locally.

With hindsight, maybe it was just a question of pinning the script-referenced nugets to a working version instead of always restoring the latest one, but I didn't try to go back after I got it working with an fsproj. It seems more robust to me that way.

I kept the build changes in their own commit so they can easily be removed if desired...

[MangelMaxime]

Using a fsproj for the build is fine to me, but could you please add a build.bat and build.sh scripts for making it easier to invoke the build ?

[et1975]

Please open an issue proposing the switch fsx -> fsproj instead of taking liberties, let's get this PR to building without the switch.
Build infra is pain to maintain, but there was recent update in elmish core that could be replicated here and across other elmish org repos.

[mlaily]

Please open an issue proposing the switch fsx -> fsproj instead of taking liberties, let's get this PR to building without the switch. Build infra is pain to maintain, but there was recent update in elmish core that could be replicated here and across other elmish org repos.

No problem, I removed the commit.

EDIT: looks like pinning the build.fsx dependencies versions was enough to fix the build.

[mlaily]

Thank you!

[et1975]

Thanks, published as 4.1.0