basic support for non-editable parts

e.g. pills, they get deleted when any character of them is removed
later on we also shouldn't allow the caret to be set inside of them

src/editor/parts.js

@@ -91,6 +91,10 @@ class BasePart {
     get text() {
         return this._text;
     }
+
+    get canEdit() {
+        return true;
+    }
 }
 
 export class PlainPart extends BasePart {
@@ -165,6 +169,10 @@ class PillPart extends BasePart {
                node.childNodes.length === 1 &&
                node.childNodes[0].nodeType === Node.TEXT_NODE;
     }
+
+    get canEdit() {
+        return false;
+    }
 }
 
 export class RoomPillPart extends PillPart {