Adds a new update-excerpts command that uses code_excerpter and code_excerpt_updater to update README code snippets from actual source, which will avoid issues with README code becoming stale (not being updated for changes like NNBD, not reflecting changes to the package API), not following expected style (formatting, lint rules), or being flat out wrong (e.g., a typo that prevents compiling). Also includes a --fail-on-change flag like the autoformat command, so that CI can validate that source code and README targets stay synchronized.

Unlike the flutter/website use of these builders, they are configured per-package here to minimize the amount that someone has to learn about repository structure in order to write correct source file references, since they are entirely package-local.

• Includes a conversion of camera's README code snippets as a proof of concept.
• Does not include enforcement of using excerpts for code snippets, but that can be added as on option to readme-check in a follow-up.

Implements flutter/flutter#89987 for this repository; it can be rolled out to flutter/packages in a follow-up

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.