♻️ Rename primitive to primitive-union

`getMatcher` / `getMatcherW` become `getPrimitiveUnionMatcher` /
`getPrimitiveUnionMatcherW`; `src/primitive.ts` and its test move to
`src/primitive-union.*`. The `Union` suffix mirrors `getTaggedUnionMatcher`.

src/index.ts

@@ -1,4 +1,7 @@
-export { getMatcher, getMatcherW } from "./primitive.ts";
+export {
+    getPrimitiveUnionMatcher,
+    getPrimitiveUnionMatcherW,
+} from "./primitive-union.ts";
 export {
     getTaggedUnionMatcher,
     getTaggedUnionMatcherW,