Use warn for printing #warning messages in JS libraries (#23786)

Previously we were only writing the message to stderr, and not recording
it as an actual warning. After this change, such messages are now
reported as warnings and controllable using `-Werror`, etc.

src/parseTools.mjs

@@ -180,7 +180,7 @@ export function preprocess(filename) {
           showStack.pop();
         } else if (first === '#warning') {
           if (showCurrentLine()) {
-            printErr(
+            warn(
               `${filename}:${i + 1}: #warning ${trimmed.substring(trimmed.indexOf(' ')).trim()}`,
             );
           }

test/test_other.py

@@ -4919,6 +4919,13 @@ def test_jslib_warnings(self):
     self.assertNotContained('This warning should not be present!', proc.stderr)
     self.assertContained('warning_in_js_libraries.js:5: #warning This is a warning string!', proc.stderr)
     self.assertContained('warning_in_js_libraries.js:7: #warning This is a second warning string!', proc.stderr)
+    self.assertContained('emcc: warning: warnings in JS library compilation [-Wjs-compiler]', proc.stderr)
+
+    err = self.expect_fail([EMCC, test_file('hello_world.c'), '--js-library', test_file('warning_in_js_libraries.js'), '-Werror'])
+    self.assertNotContained('This warning should not be present!', err)
+    self.assertContained('warning_in_js_libraries.js:5: #warning This is a warning string!', err)
+    self.assertContained('warning_in_js_libraries.js:7: #warning This is a second warning string!', err)
+    self.assertContained('emcc: error: warnings in JS library compilation [-Wjs-compiler] [-Werror]', err)
 
   # Tests using the #error directive in JS library files
   def test_jslib_errors(self):