resize the brush editor's text area whenever the brush editor is resized

this can now happen if you use the resize handle

static/brush-editor.js

@@ -31,6 +31,7 @@ export class BrushEditor extends HTMLElement {
         });
         requestAnimationFrame(() => this.#resizeTextArea());
         document.fonts.addEventListener("loadingdone", () => this.#resizeTextArea());
+        new ResizeObserver(() => this.#resizeTextArea()).observe(this);
 
         this.errorHeader = this.appendChild(document.createElement("h1"));
         this.errorHeader.classList.add("error-header");