feat(webp): WebP Image Export Implementation

[dodok8]

Description

This PR implements WebP image export functionality by adding a new toWebp function to the library. The implementation includes:

1. A new toWebp function that converts HTML nodes to WebP images
2. Support for quality parameter configuration for WebP compression
3. Comprehensive unit tests for the newly added functionality
4. Documentation updates to help users utilize the WebP export feature

Motivation and Context

WebP is a modern image format that offers superior compression while maintaining good quality, making it widely adopted across the web. This PR addresses the feature request in issue #465 to add native WebP support to the library.

While users could previously work around this limitation using toBlob with { type: "image/webp" }, the new dedicated toWebp function provides a more convenient and intuitive API for WebP exports, consistent with other format-specific functions in the library.

Usage example:

// Basic usage
toWebp(node).then(dataUrl => console.log(dataUrl));

// With quality option (0-1)
toWebp(node, { quality: 0.8 }).then(dataUrl => console.log(dataUrl));

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Enhancement (changes that improvement of current feature or performance)
• Refactoring (changes that neither fixes a bug nor adds a feature)
• Test Case (changes that add missing tests or correct existing tests)
• Code style optimization (changes that do not affect the meaning of the code)
• Docs (changes that only update documentation)
• Chore (changes that don't modify src or test files)

Self Check before Merge

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[biiibooo]

👋 @dodok8

💖 Thanks for opening this pull request! 💖

Please follow the contributing guidelines. And we use semantic commit messages to streamline the release process.

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• fix: don't overwrite prevent_default if default wasn't prevented
• feat: add graph.scale() method
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[codecov]

Codecov Report

Attention: Patch coverage is 80.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 66.50%. Comparing base (d9b2fcf) to head (9533161).
Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
src/index.ts	80.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #530   +/-   ##
=======================================
  Coverage   66.50%   66.50%           
=======================================
  Files          10       10           
  Lines         612      612           
  Branches      150      150           
=======================================
  Hits          407      407           
  Misses        144      144           
  Partials       61       61           

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