[mustache_template] Emoji support

[davidmartos96]

Support emojis in templates. The code was relying on String.length, when doing substring, which could be wrong when emojis are used.

Fixes flutter/flutter#176180

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[gemini-code-assist]

Code Review

This pull request adds emoji support to the mustache template parser by correctly handling runes instead of UTF-16 code units. The changes in scanner.dart correctly use runes for parsing, and new tests have been added to verify emoji handling.

I've identified a performance issue with the current approach in scanner.dart and suggested a more memory-efficient implementation. Additionally, I found a small error in one of the new tests regarding character offsets.

As a side note for future improvements, while parsing is fixed, the error reporting in TemplateException might still have issues with emojis, as it appears to use code unit-based offsets. This could be a good follow-up to ensure full emoji support across the package.