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.

Empieza rastreando el símbolo existente en el margen de errores y el comportamiento al pasar el cursor, que el issue identifica como referencia previa, y luego encuentra dónde se evalúan los selectores CSS en el editor. Se considera terminado cuando los selectores que coinciden con cero elementos de la página reciben el tratamiento de advertencia propuesto, comenzando con un símbolo en el margen y un texto explicativo al pasar el cursor.