sourcebundle: remove the option to append anything into a diagnostic that will then panic

[liamcervante]

This PR removes the Append function from the diagnostics supported by the source bundle package.

This function accepts any argument, but will then panic if the user passes in an unsupported type. The only supported types are the Diagnostic type and the Diagnostics type, both of which can be handled normally via the inbuilt append function. I think this function exists solely to allow a non-compile time safe way of appending diagnostics which isn't actually a useful thing to have or support.

Note, this is a breaking change so when upgrading callers will need to update their usage of these diagnostics to use the append function instead of the now-removed Append function.