Metavariables for literals

[abhinav]

We should support metavariables to match on literals. That is, to match on "foo" and 42 below.

x := "foo"
const y = 42

This will make it possible to write patches that detect, say, hard-coded strings and turn them into constants or optimize them (moving compilation of a hard-coded regex from a hot path to a top-level variable, for example).