[mustache_template] Add example app and code excerpts

[saudademjj]

Adds a runnable example application and migrates all inline README code blocks to the <?code-excerpt?> system for CI validation.

• Creates example/lib/main.dart demonstrating basic template rendering, nested paths, and lambdas
• Creates example/lib/readme_excerpts.dart with #docregion markers for all 8 README code examples
• Modernizes code style: var → final/const, adds trailing commas, fixes syntax errors in original examples
• Removes duplicate LambdaRenderBar example that was repeated in the original README
• Removes mustache_template from temp_exclude_excerpt.yaml

Fixes flutter/flutter#183936

Version-exempt: changes only affect unpublished example app (publish_to: none) and README documentation.

Test-exempt: documentation excerpt migration and example app addition only; all existing tests pass. Covered by "only affects comments or documentation" exemption.

Pre-Review Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the AI contribution guidelines and understand my responsibilities, or I am not using AI tools.
• I read the Tree Hygiene page, which explains my responsibilities.
• I read and followed the relevant style guides and ran the auto-formatter.
• I signed the CLA.
• The title of the PR starts with the name of the package surrounded by square brackets, e.g. [shared_preferences]
• I linked to at least one issue that this PR fixes in the description above.
• I followed the version and CHANGELOG instructions, using semantic versioning and the repository CHANGELOG style, or I have commented below to indicate which documented exception this PR falls under¹.
• I updated/added any relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or I have commented below to indicate which test exemption this PR falls under¹.
• All existing and new tests are passing.

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Footnotes

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[flutter-dashboard]

It looks like this pull request may not have tests. Please make sure to add tests or get an explicit test exemption before merging.

If you are not sure if you need tests, consider this rule of thumb: the purpose of a test is to make sure someone doesn't accidentally revert the fix. Ask yourself, is there anything in your PR that you feel it is important we not accidentally revert back to how it was before your fix?

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[gemini-code-assist]

Code Review

This pull request adds a runnable example application for the mustache_template package and migrates the code examples in README.md to use the <?code-excerpt?> system for CI validation. The changes also modernize the code style in the examples, fix existing errors in the README snippets, and remove a duplicated example. Overall, these changes improve the documentation and provide a helpful, runnable example for users. I have a couple of suggestions to align the new example code with the Dart style guide.

[saudademjj]

Closing to resubmit with code quality improvements based on review feedback.