Configurable syntax support

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).