feat: add `toWebp()` function

[StyleShit]

@bubkoo can you please review this?

This PR adds support for creating WebP screenshots

[vivcat[bot]]

👋 @StyleShit

💖 Thanks for opening this pull request! 💖

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

Examples of commit messages with semantic prefixes:

• fix: don't overwrite prevent_default if default wasn't prevented
• feat: add graph.scale() method
• docs: graph.getShortestPath is now available

Things that will help get your PR across the finish line:

• Follow the TypeScript coding style.
• Run npm run lint locally to catch formatting errors earlier.
• Document any user-facing changes you've made.
• Include tests when adding/changing behavior.
• Include screenshots and animated GIFs whenever possible.

We get a lot of pull requests on this repo, so please be patient and we will get back to you as soon as we can.

[vivcat[bot]]

@StyleShit Please provide us with more info about this pull request.

[codecov[bot]]

Codecov Report

Patch coverage: 80.00% and no project coverage change.

Comparison is base (b751cbf) 62.93% compared to head (2eec6e4) 62.93%. Report is 2 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #424 +/- ## ======================================= Coverage 62.93% 62.93% ======================================= Files 10 10 Lines 580 580 Branches 143 143 ======================================= Hits 365 365 Misses 151 151 Partials 64 64 ``` | [Files Changed](https://app.codecov.io/gh/bubkoo/html-to-image/pull/424?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=%E5%B4%96) | Coverage Δ | | |---|---|---| | [src/index.ts](https://app.codecov.io/gh/bubkoo/html-to-image/pull/424?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=%E5%B4%96#diff-c3JjL2luZGV4LnRz) | `89.47% <80.00%> (ø)` | |

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