LSP server needs a way to generate project change when compilation options change

[mikadumont]

In EnableNullableRefactoringProvider, we provide a code action that allows the user to control the nullable in csproj file.
However, the lsp server doesn't knon how to change project file.

Ideally we need the project system to told us how to generate text edits for the csproj file, and we can tell the client to modify it.