Walid I checked out your code and have a remark and a few suggestions.
First, I see the logic checks for the hostname being anywhere in the URL, so this means a link like https://external.tld/page?domain=flarum.tld will not open externally. You could use the URL javascript object to parse the href value and compare if the .host or .origin values match instead of String.contains().
Second I saw some redundant code. The check for /admin should not be necessary because you only add the code to the forum bundle. Same for element instanceof HTMLAnchorElement, I'm pretty sure it's redundant because the query selector a already enforces it.
And a note to users of the extension, the way it's implemented will take precedence over anything defined by another extension. Any extension defining a target or rel attribute at the TextFormatter level will be overridden. The extension will also strip the rel=ugc attribute that Flarum sets by default on user-generated content.
