JSR package for depending on all of std?

It would be nice if there was a JSR package for depending on all of std. This would allow the following pattern that was previously allowed.

For example:

```json
{
"imports": {
"$std/", "jsr:/@std/std@0.215/"
}
}
```

Then the "@std/std" package (or whatever name) would contain a deno.json like (this would probably be code generated):

```jsonc
{
"name": "@std/std",
"version": "0.215.0",
"exports": {
"./assert": "./assert/mod.ts",
"./assert/assert_equals": "./assert/assert_equals.ts",
// etc...
}
}
```

The individual files would then contain something like:

```ts
// mod.ts
export * from "jsr:@std/assert@1";
```

The downside would be these extra files, but probably not that bad for a first start. In the future, we may want to consider allowing JSR specifiers directly in an export value.

This solution would still be about as efficient as using the package directly since JSR only downloads the exports you use.