feat(vite): add templateTransform hook for template preprocessing

[BenjaminDobler]

Summary

• Adds an optional templateTransform callback to PluginOptions that transforms template content after reading from disk but before compilation
• Applied in both resolveResources() (initial build) and the HMR recompilation endpoint, so behavior stays consistent
• Non-breaking: the callback is optional and defaults to no-op

Closes #149

Usage

angular({
  templateTransform: (content, filePath) => {
    // Add data-testid attributes, inject source location metadata, etc.
    return transformedContent
  },
})

Test plan

• Verify external templates are transformed during initial compilation
• Verify HMR updates apply the same transform to fresh template content
• Verify behavior is unchanged when templateTransform is not provided

Happy to add tests if desired — the change is minimal (~6 lines of logic) and the repo doesn't currently have plugin-level unit tests.

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