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.

まず、issue が先行例として挙げている既存のエラーガターシンボルとホバー動作を追跡し、次にエディター内で CSS セレクターが評価される場所を見つけます。セレクターがページ上の要素に 1 つも一致しない場合に、提案された警告処理が適用され、まずガターシンボルと説明用のホバーテキストが表示されれば完了です。