Editable regions

One thing that would be extremely useful for embedded editors (see #1248) is defining regions of the source that should be considered editable, and others that should be considered fixed. This could be done in CodeMirror by adding a listener for key presses, which would check if the current selections and insertion points are all in editable regions, and set the read-only bit on the document accordingly. At the same time, the lexer could set a special style on tokens in the non-editable range, and that could be associated with a CSS style that grays out these areas, so the student sees which sections of the code are editable.

Empieza examinando la integración de CodeMirror para editores incrustados y, después, sigue cómo se conectan el manejo de las pulsaciones de teclas, el lexer y los estilos CSS. Define cómo se especifican las regiones editables y cómo deben funcionar el comportamiento de solo lectura y el estilo visual; como el issue no menciona archivos ni pruebas, primero es necesario investigar el proyecto para determinar la ruta de implementación.