[image_picker] Fix pickMultiImage throwing an error on iOS 13 and below.

[BeMacized]

pickMultiImage currently throws an exception on iOS 13 and below, due to it returning a string rather than a list of strings as it should. This PR fixes this behaviour.

Relevant issue:

• [image_picker][iOS]: pickMultiImage throws an invalid type cast exception on iOS 12 flutter#100132

Pre-launch Checklist

• I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
• I read the [Tree Hygiene] wiki page, which explains my responsibilities.
• I read and followed the [relevant style guides] and ran [the auto-formatter]. (Unlike the flutter/flutter repo, the flutter/plugins repo does use dart format.)
• I signed the [CLA].
• The title of the PR starts with the name of the plugin surrounded by square brackets, e.g. [shared_preferences]
• I listed at least one issue that this PR fixes in the description above.
• I updated pubspec.yaml with an appropriate new version according to the [pub versioning philosophy], or this PR is [exempt from version changes].
• I updated CHANGELOG.md to add a description of the change, [following repository CHANGELOG style].
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is [test-exempt].
• All existing and new tests are passing.

[stuartmorgan]

Oops, I meant to assign that bug to myself yesterday; I'm trying out a Pigeon migration for this implementation as a proof-of-concept for Pigeon preventing this kind of problem. I'm going to hold off on reviewing this for another day or to see how that goes, since I expect it will be a more future-proof fix. If it doesn't work out we can just patch it this way.

[stuartmorgan]

Closing in favor of #5285. Sorry for the duplicated effort here.