Wire multimethod runtime into playground

Add squint-cljs/src/squint/multi.js to the playground's importmap so
compiled code using defmulti/defmethod can resolve the runtime module,
and extend the bb init copy list to stage multi.js + internal.js
alongside the existing runtime files in public/public/src/squint/.
internal.js is not in the importmap — core.js and multi.js reach it
via a relative import, so it just needs to sit next to them on disk.

Author: borkdude

playground/bb.edn

@@ -6,7 +6,7 @@
                   (fs/create-sym-link "public/js/squint-local" (fs/parent (fs/parent (fs/absolutize ".")))))
                 (let [squint-prod (fs/file "public" "public" "src" "squint")]
                   (fs/create-dirs squint-prod)
-                  (doseq [source-file ["core.js" "string.js" "set.js" "html.js" "math.js" "test.js"]]
+                  (doseq [source-file ["core.js" "internal.js" "string.js" "set.js" "html.js" "math.js" "multi.js" "test.js"]]
                     (fs/copy (fs/file  ".." "src" "squint" source-file)
                              squint-prod
                              {:replace-existing true}))))}

playground/public/index.html

@@ -18,6 +18,7 @@
           "squint-cljs/src/squint/set.js": "./src/squint/set.js",
           "squint-cljs/src/squint/html.js": "./src/squint/html.js",
           "squint-cljs/src/squint/math.js": "./src/squint/math.js",
+          "squint-cljs/src/squint/multi.js": "./src/squint/multi.js",
           "squint-cljs/src/squint/test.js": "./src/squint/test.js",
           "@codemirror/language": "https://ga.jspm.io/npm:@codemirror/[email protected]/dist/index.js",
           "@codemirror/lang-javascript": "https://ga.jspm.io/npm:@codemirror/[email protected]/dist/index.js",