test: basic render testing on react versions

Given we have dual react 18 & 19 support and two examples(https://github.com/GrapesJS/react/pull/20), it would be worth adding a basic test that visits each example page, and asserts that grapesjs is rendered on the page. The goal would not be to test functionality but to assert that it is rendered and that's enough.

Inspect the two example pages referenced by pull request #20 and determine how each page can be visited under the React 18 and React 19 setups. Add basic render checks that assert GrapesJS appears on both pages, without testing functionality; done means both examples pass those checks for both supported React versions.

索引モデルが issue の本文から書いたものです。