refactor(share_plus)!: two-package federated architecture

[nohli]

Description

Refactor share_plus from the federated architecture to a platform interface and a package with all platform implementations.

Related Issues

#1226

Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I titled the PR using Conventional Commits.
• I did not modify the CHANGELOG.md nor the pubspec.yaml files.
• All existing and new tests are passing.
• The analyzer (flutter analyze) does not report any problems on my PR.

Breaking Change

Does your PR require plugin users to manually update their apps to accommodate your change?

• Yes, this is a breaking change (please indicate that with a ! in the title as explained in Conventional Commits).
• No, this is not a breaking change.

[nohli]

@miquelbeltran maybe the same Windows build issue as here

[miquelbeltran]

this windows build seems fixed now as well. I am not happy with the naming (some files have _windows_, some don't) but it works.

[nohli]

You're a champ 💪🏽