Highlight jQuery selectors that match 0 elements

Related to #1355 , but probably harder.

If the user's JS code calls jQuery with a selector that doesn't match any elements on the page, there's probably a mistake in their code. Whatever they do with that jQuery collection afterwards (trying to change something or attaching a click handler) will have no effect, but jQuery won't error on it because it's perfectly valid to attach a click handler to a collection with zero elements. These kinds of bugs are hard to find, and they're frequent, because students often typo IDs or forget to prefix with a #.

We should highlight these selectors the same way as #1355 (a warning symbol in the gutter and, if feasible, a squiggly yellow/orange underline).

Capturing these might be tricky. One approach could be to use static code analysis to find code that looks like $("some string") and check whether the string is a syntactically valid selector that matches zero elements. We'd have to be careful here, because $("<div>") is not a valid CSS selector but is a valid thing to pass to jQuery. Another approach could be to detect these at run-time by monkey-patching jQuery (i.e. make the $ function a wrapper around the actual jQuery function that first checks if the input is a zero-match selector), but that could cause us to miss broken selectors in code that isn't immediately executed.

Commencez par examiner l’issue associée #1355 et comparer les approches proposées d’analyse statique et de monkey-patching à l’exécution. Le travail est terminé lorsque les sélecteurs jQuery sans correspondance reçoivent l’avertissement demandé dans la gouttière et, si possible, un soulignement ondulé, sans traiter comme des sélecteurs les chaînes valides de création d’éléments telles que $("