fix: Properly handle annotating leading whitespace

[Muscraft]

rustc has a test for annotating a no-break space near the start of a line with whitespace following it. annotate-snippets currently fails this test because it uses std::char::is_whitespace for calculating whitespace_left, which treats no-break space as whitespace, leading whitespace_left to be larger than label_right, which causes a subtract with overflow panic. rustc gets around this by using rustc_parse::is_whitespace, which does not treat a no-break space as whitespace. That solution only masks the problem, as it would still panic when annotating whitespace chars with more whitespace following them. Instead, this PR fixes the Margin calculations to allow annotating whitespace before more whitespace.