Dotnet format should take a single file

### Description of the problem

Dotnet format is fantastic. We use it to check all the .editorconfig rules have been followed in our builds.

But really I want a version of prettier that I have for Typescript. So that I can run dotnet format on the open file every time I press 'Save', and so I could run it on just modified files as a pre-commit hook (rather than having to run it on the whole solution).

### Describe the solution I'd like
`dotnet format [filename|filename list|file glob pattern]`.