STRF-8622 Add helper for generating a 1x/2x srcset for store logo

[bookernath]

What? Why?

Add a helper to make it easy to generate 1x/2x srcsets for images that have stored dimensions.

The specific reason for creating this is to allow the store logo to look good even when it has been given an explicit pixel size; we can separate CSS pixel size from inherent pixel sizing easily with this helper.

Given an input like:

{{getImageSrcset1x2x image_with_dimensions "123x456"}}
and a StencilImage with dimensions like this:

image_with_dimensions: {
    data: "https://cdn.example.com/path/to/{:size}/image.png?c=2",
    width: 1400,
    height: 950,
}

It will produce a srcset value string like this: https://cdn.example.com/path/to/123x456/image.png?c=2 1x, https://cdn.example.com/path/to/246x912/image.png?c=2 2x

It can also create the best-possible version if the image is not big enough to support 2x, such as:

{{getImageSrcset1x2x image_with_dimensions "123x556"}}
https://cdn.example.com/path/to/123x556/image.png?c=2 1x, https://cdn.example.com/path/to/210x950/image.png?c=2 1.7086x which is still a valid srcset.

How was it tested?

---

Unit tests.

cc @bigcommerce/storefront-team

[jairo-bc]

Overall logic looks good to me. Left few suggestions on code changes

[bookernath]

@jairo-bc I've addressed your feedback, can you re-review? ♻️

[bookernath]

FYI @bigcommerce/dev-docs