Only prune when unreachable (not merely unused) and add module ignore option

[rtsao]

This changes the behavior of this plugin slightly and adds an option to ignore specific modules (i.e. never prune).

Imports will only be pruned if and only if all of the following conditions are met:

• Specifiers exist (i.e. not a importing for side effects only)
• "Unreachable" references to specifiers exist (i.e. not "unused" entirely)

This change means the hardcoded React exception is no longer needed, as scope traversal happens before the JSX transform and thus no references will exist, so it has been deleted.