Problem

r1322513 adds a new reusable Vue component for a CodeMirror editor. This works great but to fully satisfy WikiLambda's needs (T400015), we need readonly as a Vue prop, meaning it can change during an editing session. Currently, the core CodeMirror class checks the readonly property of the textarea only once, meaning we need to rebuild the entire editor if a client wants to change the read-only state.

Acceptance criteria

• Any consumer of a CodeMirror instance can change the read-only state, and the corresponding functionality is adjusted accordingly (i.e. linter "quick-fix")
• CodeMirrorEditor.vue watches the readOnly prop and updates the CodeMirror read-only state accordingly