Moves to using a minimal System loader for bundles generated by Deno.
TypeScript in 3.8 will be able to output TLA for modules, and the loader
is written to take advantage of that as soon as we update Deno to TS
3.8.
System also allows us to support `import.meta` and provide more ESM
aligned assignment of exports, as well as there is better handling of
circular imports.
The loader is also very terse versus to try to save overhead.
Also, fixed an issue where abstract classes were not being re-exported.
Fixes #2553
Fixes #3559
Fixes #3751
Fixes #3825
Refs #3301
