[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

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

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