[share_plus_web] fix various issues

[jpnurmi]

Description

• generated missing example runner for web
• share_plus_web was not endorsed in share_plus
• SharePlugin.registerWith() was creating an instance of the platform interface SharePlatform instead of SharePlugin for web
• the workaround for pure Dart desktop plugins had the kIsWeb guard in the wrong place

Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• My PR includes unit or integration tests for all changed/updated/fixed behaviors (See Contributor Guide).
• All existing and new tests are passing.
• I updated/added relevant documentation (doc comments with ///).
• The analyzer (flutter analyze) does not report any problems on my PR.
• I read and followed the Flutter Style Guide.
• I am willing to follow-up on review comments in a timely manner.

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 a breaking change in CHANGELOG.md and increment major revision).
• No, this is not a breaking change.

[miquelbeltran]

LGTM thanks!

[jpnurmi]

Thanks for the review! 👍