Consider emitting DynamicLocalVariables in Portable PDB

Our Portable PDB writer doesn't emit `PortableCustomDebugInfoKinds.DynamicLocalVariables`

These are emitted by C#. 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#L479

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

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