Moves from ts-node to babel-register, fixes the local sdk

[arcanis]

This diff does two things:

• First, we move from ts-node to babel-register for running our TS files. We don't need to typecheck them since they'll be typechecked later as part of tsc, and Babel is a generally mature tool that we already use for the Jest integration.

• Second, the local sdk now instantiates babel-register before instantiating PnPify. This allows us to run PnPify directly from its sources, without having to build it beforehand or to use an outdated build (as it would be the case if we were using a version from npm, for example).

• As an implementation detail, I've abstracted the TypeScript setup inside @berry/monorepo, so all packages can now use setup-ts-execution if needed.

Note that this diff will make our own sdk code slightly different from the one used by our consumers, since we'll need to setup babel-register before. This means that we won't be able to update it just by running yarn pnpify --sdk; we'll have to make the changes manually (or reapply the custom parts after each update).