refactor import/export code

I'd like to refactor the import/export code soon, moving some or most of it to Rust. This will probably come after the scheduling rework.

When working on this:

- [ ] we'll need to bear in mind use cases that the Special Fields add-on currently enables.
https://github.com/ankitects/anki/pull/507
- [ ] we'll need to provide some upgrade path for third-party imports like Mnemosyne and SuperMemo

Start by reading the scheduling rework context and pull request 507, then locate the current import/export implementation. Track the Special Fields use cases and third-party Mnemosyne and SuperMemo imports before defining the scope. Done would require an agreed Rust migration plan, preserved Special Fields behavior, and an upgrade path for those third-party imports.

Written by the indexing model from the issue text.