Update public API files outside visual studio

[javiercn]

Currently when we make API changes we need to update the public API file to reflect them. This can only be achieved easily through visual studio, which makes it a problem for our contributors on other platforms and for fixing this issue in customer contribution PRs

[javiercn]

Tracks dotnet/roslyn#48561

[Pilchie]

Also tagging @sharwell who I thought had some sort of host for running fixes outside of VS that we might be able to leverage.

[dougbu]

Including this in the DoI project for discussion in that context

[Pilchie]

It appears that the 5.0 builds of dotnet-format now support running arbitrary analyzers and fixes, so we should see whether that solves this need.

[Pilchie]

I'm going to try out the dotnet-format approach and see if I can write some instructions for how to use it.

[Pilchie]

Okay - this doesn't work today - blocked by dotnet/format#952. Hopefully that will be fixed before too long.