[4.0.x] Fixed #33237 -- Fixed detecting source maps in ManifestStaticFilesStorage for multiline files.

Switched regex to multiline mode in order to match per-line, rather
than against the whole file.

Thanks to Joseph Abrahams for the report.

Regression in 781b44240a.

Backport of 4816dc9428 from main

django/contrib/staticfiles/storage.py

@@ -54,7 +54,7 @@ class HashedFilesMixin:
         )),
         ('*.js', (
             (
-                r'(?P<matched>)^(//# (?-i:sourceMappingURL)=(?P<url>.*))$',
+                r'(?m)(?P<matched>)^(//# (?-i:sourceMappingURL)=(?P<url>.*))$',
                 '//# sourceMappingURL=%(url)s',
             ),
             (

tests/staticfiles_tests/project/documents/cached/source_map.js

@@ -1 +1,2 @@
 //# sourceMappingURL=source_map.js.map
+let a_variable = 1;

tests/staticfiles_tests/test_storage.py

@@ -260,7 +260,7 @@ class TestHashedFiles:
 
     def test_js_source_map(self):
         relpath = self.hashed_file_path('cached/source_map.js')
-        self.assertEqual(relpath, 'cached/source_map.9371cbb02a26.js')
+        self.assertEqual(relpath, 'cached/source_map.cd45b8534a87.js')
         with storage.staticfiles_storage.open(relpath) as relfile:
             content = relfile.read()
             self.assertNotIn(b'//# sourceMappingURL=source_map.js.map', content)