Introducing workers.new, custom builds, and improved logging for Workers

Introduced `workers.new` for instant JavaScript editor access and prototyping. Enabled custom build scripts for `wrangler` to support arbitrary JavaScript bundlers (webpack, Rollup, Parcel, esbuild, etc.) by allowing users to provide their own build script and upload directory. Enhanced `wrangler tail` with a new 'pretty' output format for improved log readability, alongside the existing JSON format.