fix(tools): fix type errors in create-element template

Commit c9bd577 enabled `isolatedDeclarations`, which introduced these
errors in `npm new` generated components:

> TS9008: Method must have an explicit return type annotation with --isolatedDeclarations.
> render() {
>
> TS9017: Only const arrays can be inferred with --isolatedDeclarations.
> static readonly styles = [styles];

Fixes #2872

Author: martinpitt

tools/create-element/templates/element/element.ts

@@ -1,4 +1,4 @@
-import { LitElement, html } from 'lit';
+import { LitElement, html, type TemplateResult } from 'lit';
 import { customElement } from 'lit/decorators/custom-element.js';
 
 import styles from '<%= cssRelativePath %>';
@@ -9,9 +9,9 @@ import styles from '<%= cssRelativePath %>';
  */
 @customElement('<%= tagName %>')
 export class <%= className %> extends LitElement {
-  static readonly styles = [styles];
+  static readonly styles: CSSStyleSheet[] = [styles];
 
-  render() {
+  render(): TemplateResult<1> {
     return html`
       <slot></slot>
     `;