Highlight CSS selectors that match 0 elements

If the user writes a CSS selector that doesn't match any elements of the page, then it's quite likely that they made a mistake/typo and that something in their project doesn't work as a result. We should highlight these selectors with:

1. A yellow/orange squiggly underline
2. A warning symbol (triangle with exclamation mark) in the gutter (like we do for errors but with a different symbol)
3. A popup with explanatory text when you hover either the symbol in the gutter (like we do for errors) or the underlined text

To start with, we could just do the gutter symbol with hover text, since we already do that for errors, so there's prior art.

首先追踪现有的错误 gutter 符号和悬停行为，issue 将其确定为先例，然后找到编辑器中评估 CSS 选择器的位置。当匹配页面中零个元素的选择器获得提议的警告处理时，即视为完成，首先添加 gutter 符号和解释性悬停文本。