Configurable syntax support

[jdalton]

Related to #90. Having the option to configure syntax support of nested imports and others allows for some nice optimizations.

For example, top-level parse is great but not practical as long as living binding isn't configurable. Ideally I'd like to have live bindings but also top-parsing when possible (say if all exports are const) but that means I'd have to punt on nested imports (since that requires a full parse regardless).

[jdalton]

If this could have a rank it would be the next feature to tackle. It takes reify the next step closer in being used as a generic esm loader and unlocks optimization possibilities. This could be tackled in steps with optional nested imports being top priority and then optional namespace exports as the secondary priority.