chore(core): changes ldml tsconfig to es2020 🙀

Also splits the root tsconfig.json into tsconfig.cjs.json and
tsconfig.esm.json. These are not typically used at present but in theory
you could do a `tsc -b <tsconfig.x.json>` and build all modules. In
practice, we still have some prebuild steps that make that a bit of a
pain.