[do not merge. this is a test] feat: Add a tool for inlining code excerpts

[antfitch]

This change adds a new tool that allows you to write code samples directly in markdown files. The tool will automatically extract the code, create a new file in the examples directory, and replace the code block with a <?code-excerpt?> tag.

This will make it easier to write and maintain documentation that includes code samples.

The new tool can be run with the following command:

dart tool/flutter_site/bin/flutter_site.dart inline-excerpts --file path/to/your/file.md

Description of what this PR is changing or adding, and why:

Issues fixed by this PR (if any):

PRs or commits this PR depends on (if any):

Presubmit checklist

• If you are unwilling, or unable, to sign the CLA, even for a tiny, one-word PR, please file an issue instead of a PR.
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• This PR follows the Google Developer Documentation Style Guidelines—for example, it doesn't use i.e. or e.g., and it avoids I and we (first-person pronouns).
• This PR uses semantic line breaks
  of 80 characters or fewer.

[antfitch]

@parlough and @Sfshaza I'm not going to push the change to this file, but we really need to get something like this added soon. @parlough essentially, whenever someone use gemini to work with our site, this will give gemini rules to follow (like keep lines under 80 characters, etc) and a description of what tools are available for it to look at.

For community members who might read this, we probably need something similar for other models. If someone loves making these, it would be awesome to see what you create.