QmlJSEditor: block non Utf8 codecs

Additionally adjust the postioning of the initial setCodec to all other
forced codecs.

Change-Id: Id60bfdac858851e5ad6554ff2dbcb3dd5b806c0b
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: Alessandro Portale <alessandro.portale@qt.io>

src/plugins/qmljseditor/qmljseditordocument.h

@@ -46,6 +46,8 @@ public:
     QmlJSEditorDocument(Utils::Id id);
     ~QmlJSEditorDocument() override;
 
+    bool supportsCodec(const QTextCodec *codec) const override;
+
     const QmlJSTools::SemanticInfo &semanticInfo() const;
     bool isSemanticInfoOutdated() const;
     QVector<QTextLayout::FormatRange> diagnosticRanges() const;