Consider emitting CompilationOptions into Portable PDB

[dsyme]

Our Portable PDB writer doesn't emit PortableCustomDebugInfoKinds.CompilationOptions

This information is emitted by C#, allowing a compilation to be resurrected from a PDB.

Portable PDB spec is here: https://github.com/dotnet/runtime/blob/main/docs/design/specs/PortablePdb-Metadata.md

It may be useful for F# to emit this information in some situation once we clarify what debugging experience it supports. https://github.com/dotnet/roslyn/blob/3d8acbd585a75e9cdb3509623bf6c39ceb46b2c2/src/Compilers/Core/Portable/PEWriter/MetadataWriter.PortablePdb.cs#L853

• clarify user-facing debug experience this supports
• decide if it's useful for F#
• add code to emit this if it's useful

[baronfel]

Doing this will also light up a checkmark in Nuget Package Explorer and dotnet validate experiences.

It might also enable better sourcelink-based go-to-definition experiences, because these options could be reconstituted into an accurate Project Options structure that could be used to provide actual typecheck results for that file (this is currently a problem in the kinds of views used by FSAC for sourcelink-based go-to-definition).

[baronfel]

Now that all of the major editors support fetching actual source via sourcelink, I think this should be prioritized. Emitting compiler options (which include metadata references!) into PDBs will allow for seamless navigation and editor experiences in the automatically-downloaded sources - right now once you enter into an F# source file downloaded in this way you're in a broken state in terms of editor functionality. The downloaded files don't support further navigation, proper highlighting, or intellisense - all because compilation options and references aren't available to reconstruct ProjectOptions for the file transparently on the editor side.