[mustache_template] Adds onMissingVariable to support custom rendering for unresolved variable tags

[vlidholt]

This is an addition to the mustache_template package. It adds a onMissingVariable callback to the render methods. This allows the user of the package to dynamically handle template variables that aren't predefined. This can be useful in a variety of cases.

New renderString signature:

String renderString(
  Object? values, {
  MissingVariableCallback? onMissingVariable,
});

Usage:

final String output = Template('_{{missing}}_', name: 'test_template').renderString(
  {},
  onMissingVariable: (name, context) {
    return 'bob';
  },
);

Example use cases:

• Image transformations, e.g. {{image.avatar.128.128.webp}}
• Date formatting tokens, e.g. {{date.createdAt.yMMMd.en_US.utc}}
• Money and number formatting, e.g. {{money.total.USD.en_US.2}}
• CSS class composition, e.g. {{class.button.primary.large}}
• Route or URL generation, e.g. {{route.userProfile.42.settings.mobile}}
• Icon selection by variant, e.g. {{icon.warning.red.24.filled}}
• CDN asset selection and cache busting, e.g. {{cdn.heroBanner.dark.2x.webp}}

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[gemini-code-assist]

Code Review

This pull request introduces an onMissingVariable callback to the mustache_template package, allowing for custom rendering or fallback values when variable tags are unresolved. The changes include updates to the Template interface, the internal Renderer logic to support the callback, and the addition of a MissingVariableContext class to provide metadata such as template name and source offset. The package version is bumped to 2.1.0, and comprehensive tests have been added to verify the functionality in both strict and lenient modes. I have no feedback to provide.

[stuartmorgan-g]

This repository does not have issues enabled.

Please see the repository README. (We wouldn't require issues as part of the template if we didn't have an issue tracker.)

Please do file an issue and expand there on the reason for the proposal. We generally don't want to expand the API surface of this package, and it's not clear from the PR description why a client wouldn't be able to provide values for template variables in advance.

[stuartmorgan-g]

(Marking as a draft pending an issue where the feature request is discussed and evaluated in more detail.)