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.

Start by reviewing related issue #1355 and comparing the proposed static-analysis and runtime monkey-patching approaches. The work is done when zero-match jQuery selectors receive the requested gutter warning and, if feasible, a squiggly underline without treating valid element-creation strings such as $("