[C#] Support the .cake file format

[rwols]

Cake is a C# build system. The files end with the .cake extension. Its syntax is similar, but not equal to, the C# syntax. Most likely much of the C# syntax can be re-used for this.

Notably the OmniSharp language server can work with .cake files, so it would be nice to have support for these files out-of-the-box.