Add warning for BIDI characters in page renders and in diffs (#17562)

Fix #17514

Given the comments I've adjusted this somewhat. The numbers of characters detected are increased and include things like the use of U+300 to make à instead of à and non-breaking spaces.

There is a button which can be used to escape the content to show it.

Signed-off-by: Andrew Thornton <art27@cantab.net>
Co-authored-by: Gwyneth Morgan <gwymor@tilde.club>
Co-authored-by: silverwind <me@silverwind.io>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

web_src/js/features/repo-legacy.js

@@ -10,6 +10,7 @@ import {
   initRepoIssueWipToggle, initRepoPullRequestMerge, initRepoPullRequestUpdate,
   updateIssuesMeta,
 } from './repo-issue.js';
+import {initUnicodeEscapeButton} from './repo-unicode-escape.js';
 import {svg} from '../svg.js';
 import {htmlEscape} from 'escape-goat';
 import {initRepoBranchTagDropdown} from '../components/RepoBranchTagDropdown.js';
@@ -533,6 +534,8 @@ export function initRepository() {
       easyMDE.codemirror.refresh();
     });
   }
+
+  initUnicodeEscapeButton();
 }
 
 function initRepoIssueCommentEdit() {