Port `sync-fcs-repo` and `copy-fcs-repo` to the new build system

[MangelMaxime]

Description

Hello @ncave, sync-fcs-repo target seems to be broken to me.

Our build.fsx si trying to update a build.fsx file from the FCS repo which doesn't seems to exist. And even, when trying to run commands manually, it seems like I am missing steps or not running from the correct directory if I follow what our build.fsx tell me.

What are the steps to build the service_slim and the Fable metadata packages?

What is the usage of service_slim?

[ncave]

@MangelMaxime I don't think they're broken, but they may be expecting you to have this dotnet/fsharp fork repo locally, in a specific folder. There are quite a few branches in there, but most of them are only kept for historic reasons. You can just go to the PR page, the important ones are listed as long-term open PRs for reference:

• fable (see PR #1) - Modified FCS version that can be compiled with Fable (for REPL use, etc.).
• service_slim (see PR #2) - The FCS version that Fable tool uses. Stock FCS, with a light facade on top.
• export (see PR #3) - a slightly modified FCS version used to produce .NET metadata (for REPL).

There are instructions how to build each one of those, in the PRs.

Technically, you only need to run sync-fcs-repo when there is something new in those branches. At the moment, they have already been synched with Fable. These branches are usually rebased from the upstream FCS (dotnet/fsharp) from time to time, usually whenever there is some major F# lang revision.

I plan on updating those branches close to the .NET 8.0 release, and I'll sync them with Fable after.
The service_slim and export branches are relatively easy to rebase to the latest F# version, where fable is usually a bit more involving.

It all depends on how much churn there was in dotnet/fsharp in the mean time, sometimes there is a lot.

If that's unclear or you have other questions, please feel free to ask.

And lest I forget, thanks for picking up the torch and making new releases!