feat: Add support for image srcset and source element

Changes:

Adds support for the <img> element srcset attribute.
Adds support for the <source> element.
Modified the tests to take the srcset attribute into account.

Example:

<html>
  <head>
  </head>
  <body>
    <h1>Hey!</h1>
-   <img srcset="images/my_image.webp 1x,
-                images/my_image_high_res.webp 2x,
-                https://example.com/my_external_image.webp"
-        src="images/my_image_for_safari.jpeg" />
+   <img srcset="index.2c3e74ed2f07dde5b37ea53ba02d8da4.webp 1x, 
+                index.e64db08d87e7ea59f8a01df1dcbb9de5.webp 2x,
+                https://example.com/my_external_image.webp"
+        src="index.e64db08d87e7ea59f8a01df1dcbb9de5.jpeg" />
</body></html>

Implementation notes:

Changed the output of the method createFileObject() from a single File to File[]. This is because a single html element can output multiple files:
- For example, a single <img> element can require multiple files to be bundled if the srcset attribute is used.
- If we want to support file imports or CSS modules in JavaScript, a single <script> element will need to be able to add many different files to the bundle.

Bikeshedding:

Because the <source> element can be used for both images and videos, maybe we should rename ImageAsset to something more generic like FileAsset? Or would that be too generic?

kt3k / packup

feat: Add support for image srcset and source element #39