LibJS: Infer file extensions when resolving module paths

This allows `import "./foo"` to succeed, even if the file is actually
called `foo.js`. IDEs commonly exclude file extensions in auto-imports.

Closes #14364.

Userland/Libraries/LibJS/Tests/modules/path-resolution.mjs

@@ -0,0 +1,5 @@
+// If all of these succeed, path resolution with implicit extensions is working.
+
+import "./module-with-default";
+import "./submodule";
+import "./json-module" assert { type: "json" };