feat: add declared size support for lazy files

[Ehesp]

This PR adds support for defining a lazy file with metdaata, including a size.

I've got a use-case using the GitHub tree api where I get the file names, but also the size of the file, see https://api.github.com/repos/vercel-labs/just-bash/git/trees/HEAD

When defining in-memory files, such as:

const env = new Bash({
  files: { "/data/file.txt": () => fetch('...').then(r => t.text()) },
});

When stat() (etc) is called on the FS, it performs a fetch request to return metadata (e.g size). So if the user runs a ls -la command on a directory with say 100 lazy-loaded files, this would be 100 network requests.

This PR adds support for defining the metadata upfront, so content loading is deferred until the content itself is actually needed, e.g:

    "/data/indexed.bin": {
      lazy: async () =>
        new Uint8Array(
          await (await fetch("https://example.com/indexed.bin")).arrayBuffer(),
        ),
      size: 1024,
    },

With this, the content isn't fetched until it's actually needed since we provide the metadata upfront.

Without this, I basically need to re-implement my own file system instance, re-building all of the complexity which the built in ones provided out of the box.

[vercel]

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