codegen-js: URL import declarations for JS modules

import "https://cdn.example.com/lib.js" now emits:
  - module mode: import "https://..." at the top of the generated file
  - bundle/IIFE mode: // external: https://... comment

El source imports (.el files) are excluded -- they were already inlined
by resolve_imports before codegen. Any import path that doesn't end in
.el or starts with http(s):// is treated as an external JS dependency.

el-compiler/src/codegen-js.el

@@ -1112,6 +1112,32 @@ fn codegen_js_inner(stmts: [Map<String, Any>], source: String, bundle_mode: Bool
         js_emit_blank()
     }
 
+    // URL import pass: emit `import "url"` (module mode) or a comment
+    // (bundle mode) for any import whose path starts with http(s):// or
+    // doesn't end in .el (i.e., it's a JS/CSS/CDN import, not an El source
+    // import which was already inlined by resolve_imports).
+    let n: Int = native_list_len(stmts)
+    let i = 0
+    while i < n {
+        let stmt = native_list_get(stmts, i)
+        let sk: String = stmt["stmt"]
+        if str_eq(sk, "Import") {
+            let ipath: String = stmt["path"]
+            let is_url = str_starts_with(ipath, "http://")
+            let is_url = is_url || str_starts_with(ipath, "https://")
+            let is_js = !str_ends_with(ipath, ".el")
+            if is_url || is_js {
+                if bundle_mode {
+                    js_emit_line("// external: " + ipath)
+                } else {
+                    js_emit_line("import " + js_str_lit(ipath) + ";")
+                }
+            }
+        }
+        let i = i + 1
+    }
+    js_emit_blank()
+
     // Pre-registration pass: scan all FnDefs for @async decorators so that
     // forward calls to @async functions get `await` even if the callee is
     // defined after the caller.