Use FST-1027 instead of the ugly FAKE header

[matthid]

Description

We already have an approved syntax for F# (https://github.com/fsharp/fslang-design/blob/master/tooling/FST-1027-fsi-references.md). We could implement that and get rid of the FAKE header in the following way:

#if FAKE_DEPENDENCIES
#r "paket: storage: none"
#r "paket: source https://api.nuget.org/v3/index.json"
#r "paket: nuget Fake.Core.Target prerelease"
#endif

or as an alternative:

#r "fake: Fake.Core.Target prerelease"

(First fake line would set NuGet-Gallery as source (if non exists) and storage:none)

• We have an #if block around the directives.
• We preprocess that block and don't set the FAKE_DEPENDENCIES at runtime
• (Future) we can set that flag and make the block optional once everyone supports that syntax