[pigeon] updated to 1.0.0

[gaaclarke]

Just an update to the pubspec to christen 1.0.

Pre-launch Checklist

• The title of the PR starts with the name of the package surrounded by square brackets, e.g. [shared_preferences]
• 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 Flutter Style Guide.
• I listed at least one issue that this PR fixes in the description above.
• I added new tests to check the change I am making or feature I am adding, or Hixie said the PR is test-exempt.
• I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy.
• I updated CHANGELOG.md to add a description of the change.
• I updated/added relevant documentation (doc comments with ///).
• I signed the CLA.
• All existing and new tests are passing.

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[gaaclarke]

I'm updating video_player to use 1.0 as an extra sanity check before we land this: flutter/plugins#4277

[gaaclarke]

Looks like this is good to go. video_player's example compiles and runs on iOS and Android. I shook out the last annoyances. You ready to do this @stuartmorgan ?

[stuartmorgan-g]

LGTM!