feat(spec): add keyedBy(element, tags) identity helper

Joins element.find({testTag: tag})?.text per tag with U+001F as the
delimiter so user-visible text can never collide with the separator.
Returns empty string for an undefined element.

pkg/spec/src/index.ts

@@ -23,7 +23,7 @@ export type {
   WeightedEntry,
 } from "./types.ts";
 
-export { extract } from "./extract.ts";
+export { extract, keyedBy } from "./extract.ts";
 export { always, eventually, next, now } from "./ltl.ts";
 export {
   InputText,