Node- and browser-specific implementations of blob helpers

[eritbh]

This uses the imports key in package.json to point to different inner implementations of compressBlob and decompressBlob in node vs. in other environments.

The Node-specific implementation is based on the built-in node:zlib module and uses Buffers to convert between base64/utf8 strings and binary data. It has no dependencies, and node consumers can install the package with --no-optional to avoid downloading them when installing this package.

The browser implementation depends on pako and core-js. It uses polyfills of Uint8Array#toBase64() and .fromBase64(). for binary <-> base64 conversions, and relies on pako's behavior to handle utf8 encoding/decoding.

This PR also restricts consumers to only importing stuff from the module entrypoint; reaching into subpaths manually is no longer allowed.

• README docs about the optional dependencies
  • figure out the story for installing toolbox-devvit (which should always be used in node contexts) without pulling in the optional dependencies - is specifying --no-optional still necessary if toolbox-devvit's lockfile already reflects that the optional dependencies were skipped initially? maintaining this might be tricky