fix(sandpack-template): use custom react sandpack template

[danilowoz]

This solves the issue that users can't download a runnable sandbox from CodeSandbox. Closes#6482

Since we introduced the new editor, we had to make some changes to make templates easier to maintain and make them compatible with our microVM solution. Consequently, we had to remove some custom rules applied to the sandbox files before downloading them. For example, in the react template, we used to include scripts, dependencies, missing files, and move all files to src.

With this PR, we move the responsibility of providing all files and a runnable sandbox to react.dev, instead of relying on hidden layers inside CodeSandbox download logic. As a result, react.dev can now easily upgrade/download react dependencies, provide the minimal amount of files to run a React.js project and keep it transparent, avoiding any future headache.

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Size changes

Details

📦 Next.js Bundle Analysis for react-dev

This analysis was generated by the Next.js Bundle Analysis action. 🤖

Three Pages Changed Size

The following pages changed size from the code in this PR compared to its base branch:

Page	Size (compressed)	First Load
/404	77.36 KB(🟡 +37 B)	181.17 KB
/500	77.35 KB(🟡 +37 B)	181.17 KB
/[[...markdownPath]]	78.95 KB(🟡 +37 B)	182.76 KB

Details

Only the gzipped size is provided here based on an expert tip.

First Load is the size of the global bundle plus the bundle for the individual page. If a user were to show up to your website and land on a given page, the first load size represents the amount of javascript that user would need to download. If next/link is used, subsequent page loads would only need to download that page's bundle (the number in the "Size" column), since the global bundle has already been downloaded.

Any third party scripts you have added directly to your app using the <script> tag are not accounted for in this analysis

Next to the size is how much the size has increased or decreased compared with the base branch of this PR. If this percentage has increased by 10% or more, there will be a red status indicator applied, indicating that special attention should be given to this.