fix: export StartOptions and MobileScannerViewAttributes for tests

[w3ggy]

I added StartOptions and MobileScannerViewAttributes to export to let the developers to mock the MobileScannerPlatform in tests.

I tried to write golden tests and mock the MobileScannerPlatform but I cannot do it because I cannot import classes which are necessary to override Future<MobileScannerViewAttributes> start(StartOptions startOptions)

[navaronbracke]

@w3ggy Should we mark this as @visibleForTesting ? Otherwise, I think this looks good to have.
While there aren't many people that write tests for mobile_scanner usage, there are definitely some.

See also #1042 which is also related to tests and we should look into.

[navaronbracke]

@w3ggy I went ahead and verified that we do not need @visibleForTesting here, since that allows it to be used only in tests and the defining library. The latter is not the case, as the platform interface definition uses it.

I updated your pull request with a changelog entry and fixed the import order, per the https://dart.dev/tools/linter-rules/directives_ordering rule.

Thanks for catching this!