Add configuration for MSBuild files

[ghost]

Currently, opening any MSBuild project file results in the default text language being used. At the least, the xml language should be used to gain proper highlighting. The file extensions I'm aware of are .csproj, .fsproj, .vbproj, and .proj.

It is, of course possible to work around this with :lang xml each time you open the file, and maybe other ways. However, I believe helix should provide this out of the box.

[sudormrfbin]

A PR for this would be definitely welcome, you can consult https://docs.helix-editor.com/master/guides/adding_languages.html on adding a new language entry. Re-using the xml grammar is a good starting point, and it can replaced with a dedicated tree-sitter grammar if it becomes available.

[ghost]

Related for XML support: https://github.com/eclipse/lemminx

This could bridge the gap for MSBuild project files, since a schema exists for them. I'm not sure if the schema is meant to be inferred or how that works, but that could be helpful.