Auto merge of #185 - servo:…, r=emilio
Auto merge of #185 - servo:…, r=emilio

Fix interaction of skip_whitespace and nested blocks

This fixes (at least some of) the test failures at https://github.com/servo/servo/pull/18171

In code like this:

```css
unsupported selector ! { stuff }
valid selector { color: green }
```

`rules_and_declarations::parse_qualified_rule` would leave the parser just after the first `{` with `Parser::at_start_of == Some(BlockType::CurlyBracket)`. The latter means that, unless `Parser::parse_nested_block` is called, the parser needs to skip over this block until the matching `}` before doing anything else. This PR makes `Parser::skip_whitespace` and `Parser::skip_cdc_and_cdo` (both added recently in https://github.com/servo/rust-cssparser/pull/181) take care of `Parser::at_start_of` correctly the same way `Parser::next` does.

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servo/rust-cssparser,servo/rust-cssparser