Support quick info for embedded languages

Right now when you hover over a regex/datetime/json string all you see is class System.String. That's not very helpful and there's much more useful information we could show instead (for example, what the regex item under the mouse cursor means).

For datetime/regex we already have the docs for these constructs (which we use for completion lists). So we could definitely power this with the same info.

@stephentoub also mentioned it would be useful if the regex work could potentially describe to the user what a regex means. For example:

This would be in line with what several useful regex tools offer today.