fix(share_plus): Show NSSharingServicePicker asynchronously on main thread

[Seuleuzeuh]

Description

In the MacOS implementation of share_plus show the NSSharingServicePicker asynchronously.

For testing purpose i've update the corresponding sample app :
replace filepicker_windows with file_selector, for a working sample in all desktop (MacOS, Windows, Linux).

Related Issues

• Related [Request]: Show NSSharingServicePicker asynchronously on main thread #1177

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.

[miquelbeltran]

LGTM! Thanks for updating the example code as well

[miquelbeltran]

Just a note, we need clarification if #1177 refers to iOS or macOS (or both)

[Seuleuzeuh]

Just a note, we need clarification if #1177 refers to iOS or macOS (or both)

Only macOS (the change refer to the file SharePlusMacosPlugin.swift.