wikieditor is not loaded when using the editor=wikieditor option in texarea fiels on MediaWiki 1.33.1.
The problem is in the javascript code of the PF_wikieditor.js file, because it calls mw.loader.using passing [ 'jquery.wikiEditor.toolbar', 'jquery.wikiEditor.toolbar.config' ] as the first parameter, however, it seems that in the latest release of wikieditor those modules are not loaded, I think this is the commit where this was changed: https://gerrit.wikimedia.org/r/plugins/gitiles/mediawiki/extensions/WikiEditor/+/d23d09628122881bf4bcc91d2bc56c169019dbf8.
To fix this, the line
mw.loader.using( [ 'jquery.wikiEditor.toolbar', 'jquery.wikiEditor.toolbar.config' ] , function() {
can be replaced by
$.when( mw.loader.using( 'ext.wikiEditor' ), $.ready ).then( function() {
as shown in https://www.mediawiki.org/wiki/Extension:WikiEditor/Toolbar_customization
Also, I think that the call to $.wikiEditor.isSupported can be removed. The function isSupported was removed from wikieditor (in commit https://gerrit.wikimedia.org/r/c/mediawiki/extensions/WikiEditor/+/400697), because, as stated in the commit message, "WikiEditor works in all browsers that MW supports".