Loads casc_interp.aether. Used only for a double-quoted string's content (CASC.md §7) -- see that grammar file's own moduledoc-style comment for why bare interpolated values in the main grammar don't go through here.

This sub-grammar is invoked dynamically -- once per double-quoted string literal rather than once overall -- which turns out to make Grammar.Native an even bigger win here than for the main grammar: under Grammar.VM (Cooper.Test.VMParity.run_interp_vm/1, kept for the backend-parity test/benchmark, test/support/ only), every single string literal in a parsed file re-parsed casc_interp.aether's own source text from scratch; Grammar.Native (Cooper.NativeInterpGrammar, compiled ahead of time by mix ichor.gen, the only backend this module still calls into) compiles it exactly once, ahead of Cooper's own build, regardless of how many string literals a given file has.