Pretty print XML on save

[danyill]

I'd like it very much if we pretty-printed the XML on save, in a similar manner to OpenSCD.

See: https://github.com/openscd/open-scd/blob/2706f82283cc7a58a8da5ca4cb775bfec7c6b986/src/menu/SaveProject.ts#L3C1-L14

I am constantly opening things from core plugins and then taking manual steps to format them and I think most SCL users would expect their documents to be formatted on save (albeit with some risk for Private/extension namespace elements).