JS: Isolate ad-hoc faststrings
Compare changes
This MR is the first step in cleaning up the JS EDSL to prepare for typing the dsl. It is a simply refactor moving strings that are sprinkled throughout the backend into a single file. The next steps will be to use a single type for identifiers in the backend (right now there are 3, Ident FastString and JStgExpr), and then move the identifiers into modules that make sense (just as we shouldn't have a Types god module, we should seek to avoid a Symbols god module).