`@babel/plugin-transform-typescript` removes unused imports because it assumes they're types (yuck). That's how TS behaves so it's at least understandable.

However, it doesn't seem to respect the fact that our plugin is *emitting* code that *will* use the imports.

This adds a failing test.

Builds off #31 to fix #30.

Instead of keeping *everything* as in #31 (which is not safe in general), we use the `pre` to take a snapshot of available imports and then only when we discover that a template wants to use a name that is not in scope do we check if it was in the original set of available imports and reintroduce an import for it.