Multi-line diagnostic underlining

[ReeceHumphreys]

The new error output system introduced in PR #236 generates mangled underlines for slice errors that span multiple lines. Perhaps we can solve this by allowing Diagnostic to contain an array of Span to avoid whitespace from the beginning of the new line being included in the span.

The following slice shows this error:

module MyModule {
    interface MyInterface {
        op1(i: stream
            int64, j: stream int64);
    }
}