Add 'sibling' elements that cannot be pruned individually

[connor4312]

Some prompts use XML tags like

<tag>
content
</tag>

We should ensure that the opening and closing tags cannot be pruned individually. I suggest something like

import { PromptElement, usePruneWith } from '@vscode/prompt-tsx';

class MyPromptElements extends PromptElement {
  render() {
    const token = usePruneWith();
    return <>
      <Chunk pruneWith={token}>{`<${this.props.tagName}>`}</Chunk>
      {this.children}
      <Chunk pruneWith={token}>{`</${this.props.tagName}>`}</Chunk>
    </>;
  }
}