SPIKE: brand new core

[erikyuzwa]

• aiming to turn this into a monorepo while we come up with an approach
• moved all existing code into legacy
• created a new folder for core
• core just has one Class for the moment: Being (maybe change to Entity?)
• core builds everything (with npm run build) and produces an ESM / UMD build of jsrl.js
• eventually create a new base folder for engine
• engine will use the core as a dependency
• maybe all the builds in any sub-folder will output everything to the BASE folder under dist? 🤷