Updates the upload primitive to target Solid.js 2.0 with improved error handling.

• isServer imported from @solidjs/web (moved out of solid-js/web)
• createDropzone: onMount/onCleanup replaced with direct listener attachment in setRef using getOwner/runWithOwner for cleanup
• fileUploader moved to its own file (src/fileUploader.ts) and converted to a ref callback factory
• @solid-primitives/utils dependency removed (inlined the one usage)
• Both createFileUploader and createDropzone now expose an error: Accessor<unknown> signal that captures callback rejections reactively
• fileUploader accepts an optional onError callback; without it, rejections propagate as unhandled
• Tests expanded from 1 to 19 covering initialisation, mutations, error paths, and event dispatch

Breaking changes:

• use:fileUploader directive removed — use ref={fileUploader(opts)} instead
• Requires solid-js@^2.0.0-beta.7 and @solidjs/web@^2.0.0-beta.7