Fix console error caused by embed-gist-via-iframe (#2539)

Co-authored-by: Federico Brigante <github@bfred.it>

Author: kidonng

source/index.ts

@@ -328,6 +328,10 @@ export const _isSingleFile = [
 	'https://github.com/sindresorhus/refined-github/blob/fix-narrow-diff/distribution/content.css',
 	'https://github.com/sindresorhus/refined-github/blob/master/edit.txt'
 ];
+export const isSingleGist = (): boolean => isGist() && /^\/(gist\/)?[^/]+\/[0-9a-f]{32}$/.test(location.pathname);
+export const _isSingleGist = [
+	'https://gist.github.com/sindresorhus/0ea3c2845718a0a0f0beb579ff14f064'
+];
 
 export const isTrending = (): boolean => location.pathname === '/trending' || location.pathname.startsWith('/trending/');
 export const _isTrending = [