Add option to pad an image to a square

[BritishWerewolf]

This PR is stacked on PR 971.
On my fork, this was branched off the other PR, but I couldn't find a way to do this with PRs - sorry 😅

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It's common to want to create a square image, currently we need to do some manual calculations.

With this PR, we can do something like this, with no arguments to just fill the blank area.

const image = await RawImage.fromURL("https://picsum.photos/600/400");
const resized = await image.resize(300, null); // 300x200
const padded = await resized.padToSquare();    // 300x300

Or we can provide a value, and make the image pad to that size.

const image = await RawImage.fromURL("https://picsum.photos/600/400");
const resized = await image.resize(300, null); // 300x200
const padded = await resized.padToSquare(400); // 400x400

Providing a value that is smaller than either the width or height will just return the image unchanged.

const image = await RawImage.fromURL("https://picsum.photos/600/400");
const resized = await image.resize(300, null); // 300x200
const padded = await resized.padToSquare(100); // 300x200