feat: show diagnostics on multiline diags

[Kazy]

Currently if a diagnostic spans multiple lines, the current implementation doesn't show anything past the first line. This changes this behavior.

This is set as the default, but happy to but that behind a flag if needed.

The one case that is problematic (even now) is if end_col doesn't exist (as it seems it can be, same for end_lnum) then the last check diag.col <= col and (diag.end_col or diag.col) >= col) will only be true on the actual col and nowhere else since (diag.col <= col and diag.col >= col) is equivalent to the two being equal. I don't know how often this happens in practice. This is why I've kept the old behavior if we don't have diag.end_col and diag.end_lnum, though personally I would show it for the whole line if we don't have a end_col.