feat: allow user to set TOC display preference (#6943)

Co-authored-by: Tom Moor <tom.moor@gmail.com>

shared/editor/styles/EditorStyleHelper.ts

@@ -36,4 +36,7 @@ export class EditorStyleHelper {
 
   /** Minimum padding around editor */
   static readonly padding = 32;
+
+  /** Table of contents width */
+  static readonly tocWidth = 256;
 }