Support VS format document command

[decriptor]

Once I added the extension, I was no longer able to format my csproj files.

[mhutch]

This is because the MSBuild Editor is not built on top of Visual Studio's XML editor - it uses its own completely independent XML editor, which does not yet support the Format Document command. I intend to add this but I'd like to support editorconfig first so it's possible to control the formatting behavior.