feat(all): add Compiler.is* target detection flags for conditional branching

[MangelMaxime]

Fable's conditional compilation relies on compiler directives. It is working fine but has a huge limitations.

In IDE, you often end up with no IDE support most of the compiler directives branches as the compiler directives is not set in the fsproj. And even when setting it up in the fsproj the experience is not always great depending on the IDEs.

This PR is building on top of the existing foundation in Fable used for Compiler.debugMode or Compiler.triggeredByDependency.

The idea is that in Fable, if a if ... then ... else ... structure is using boolean constant in their condition then Fable auto erase unreachable branches.

This now allow us to write things like

open Fable.Core

let greet name =
    if Compiler.isJavaScript then
        $"Hello from JS, {name}!"
    elif Compiler.isPython then
        $"Hello from Python, {name}!"
    else
        $"Hello from .NET, {name}!

which compiles to

JavaScript

 export function greet(name) {
      return `Hello from JS, ${name}!`;
  }

Python

def greet[_A](name: _A=UNIT) -> str:
      return ("Hello from Python, " + str(name)) + "!"

The only impact is on .NET where there will be a real runtime check against the boolean. This should be more than fast enough and people can use Compiler.isDotnet to avoid having to check all the conditions.

Summary

Thanks to these changes, we are now able to provide a better experience for interop/universal code.

[github-actions]

Python Type Checking Results (Pyright)

Metric	Value
Total errors	34
Files with errors	4
Excluded files	4
New errors	✅ No

Excluded files with errors (4 files)

These files have known type errors and are excluded from CI. Remove from pyrightconfig.ci.json as errors are fixed.

File	Errors	Status
temp/tests/Python/test_hash_set.py	18	Excluded
temp/tests/Python/test_applicative.py	12	Excluded
temp/tests/Python/test_nested_and_recursive_pattern.py	2	Excluded
temp/tests/Python/fable_modules/thoth_json_python/encode.py	2	Excluded

[MangelMaxime]

Hello @ncave,

my memory is failing.

In order to make, fable-standalone tests to pass I need to regenerate the fable-metadata for Fable.Core.

I don't remember how to do that, is it as simple as copying the result of dotnet build -c Release over their ?

[ncave]

@MangelMaxime
Fable.Core doesn't actually have code, just bindings/metadata, so you can just use it as is. Just replace the one in fable-metadata with latest (as trimming it will not save much anyway).