feat: use rust native webp en/decoding

[nekowinston]

Still a WIP, based on the recently released (and even more recently fixed) image-webp crate.

Couple of notes:

• I opted for adding webp-pure as a mutually exclusive feature rather than a full replacement, as image-webp currently only supports lossless encoding.
• This bumps the MSRV up to v1.67.1 because of image-webp dependencies.
• I moved the COLORS static in ./tests/test_png.rs to a common module, since three files are now using the same contant, and using mod png; would also always run png tests when just testing with cargo test --test test_webp-pure --features all-pure for me.

We should probably wait for the release of image-webp v0.1.2/v0.2.0; I mainly wrote this PR for https://github.com/catppuccin/toolbox/pull/147, to compile that project to WASM with pure Rust webp support, without having to rely on ImageMagick.

Summary by Sourcery

Add support for Rust-native WebP encoding and decoding with the image-webp crate, introducing a webp-pure feature for lossless encoding. Update the MSRV to v1.67.1 and refactor tests to use a common module for shared constants. Update documentation and add new tests for WebP functionality.

New Features:

• Introduce support for WebP encoding and decoding using the Rust-native image-webp crate, with a new webp-pure feature for lossless encoding.

Enhancements:

• Refactor the test suite to use a common module for shared constants, improving code reuse and maintainability.

Build:

• Update the Minimum Supported Rust Version (MSRV) to v1.67.1 to accommodate dependencies of the image-webp crate.

Documentation:

• Update the README to reflect the new MSRV and document the new webp-pure feature alongside existing image encoding features.

Tests:

• Add new tests for WebP encoding and decoding, including both static and animated images, to ensure the correctness of the new webp-pure feature.

Chores:

• Add a rust-toolchain.toml file to specify the Rust toolchain version and components required for the project.

[sourcery-ai[bot]]

Reviewer's Guide by Sourcery

This pull request introduces native Rust WebP encoding and decoding support using the image-webp crate, as an alternative to the existing libwebp-sys2 implementation. The changes include new pure Rust WebP encoder and decoder implementations, updated tests, and modifications to the feature system to support both WebP implementations.

File-Level Changes

Change	Details	Files
Implement pure Rust WebP encoding and decoding	Add new webp_pure module with WebP encoder and decoder implementations Update feature system to support both webp and webp-pure features Implement WebPStaticEncoder, WebPDecoder, and WebPSequenceDecoder structs Add error handling for mutually exclusive webp and webp-pure features	src/encodings/mod.rs src/encodings/webp_pure.rs src/format.rs
Update tests for new WebP implementation	Add new test file for pure Rust WebP implementation Update existing WebP tests to use new reference images Implement tests for both lossless and lossy WebP encoding/decoding	tests/test_webp.rs tests/test_webp-pure.rs
Refactor test structure and common code	Move COLORS constant to a new common module Update test files to use the new common module	tests/common/mod.rs tests/test_png.rs tests/test_gif.rs
Update project metadata and documentation	Bump MSRV to 1.67.1 Update README with new WebP feature information Add rust-toolchain.toml file	README.md rust-toolchain.toml

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