This configures the existing build tasks to use esbuild by default. If
using the plain files is desired, passing `--bundle=false` will build
using plain files and still produce a runnable system.
This is only a basic build; a more efficient build is provided later
when gulp is replaced by hereby.
Although the existing require calls would work fine, it's more
consistent to make them actual imports and let them be converted at
compile/bundle time. It also lets us emit ESM if needed.
There are still conditional requires present for things like the node
System, but that's a more difficult problem to solve.
