[JEWEL-1267] Support sized images

[AlexVanGogen]

Summary

Add support for parsing and rendering sized images in Jewel Markdown when image dimensions are provided via HTML width and height attributes or native Markdown image attribute blocks ({width=100 height=50%}), including both absolute pixel values and percentage-based dimensions with proportional scaling when only one dimension is specified.

Fixes JEWEL-1267

Changes

• New DimensionSize sealed interface with Pixels and Percent value classes to represent image dimension specifications
• Extended InlineMarkdown.Image with optional width and height properties
• Added parsing of Markdown image attribute blocks: ![alt](url){width=100 height=50%} — supports unquoted, single-quoted, and double-quoted attribute values, with px and % suffixes
• Added parseHtmlSizeValue() for parsing HTML size attributes (e.g., "100", "100px", "50%") into DimensionSize values, with negative percentage values treated as invalid
• Refactored Coil3ImageRendererExtensionImpl to compute placeholder sizes based on specified dimensions — when both are given they're used directly, when only one is specified the other scales proportionally, and when none are specified the original image size is used
• Updated api-dump-experimental.txt with new DimensionSize API entries
• Added comprehensive unit tests:
  • 10 tests for Markdown image attribute block parsing
  • 12 tests for Compose image rendering with dimensions
  • Extended HTML converter tests for <img> tag parsing

Screenshots

HTML

Percentage and pixel dimensions (50%, 540px/304px)

Mixed dimension combinations (pixel+percentage, percentage+pixel)

Markdown

Original images

Using sized width/height

Release notes

New features

• Added support for sized images in Markdown rendering — HTML <img> tags with width and height attributes and native Markdown image attribute blocks ({width=100 height=50%}) are now parsed and rendered at the specified dimensions, with proportional scaling when only one dimension is provided ([JEWEL-1267] Support sized images #3442)

[AlexVanGogen]

Created using a skill from here :P

(except for images, those had to be added manually)

[AlexVanGogen]

TIL some preview providers also support given width and height: https://about.gitlab.com/releases/2022/12/22/gitlab-15-7-released/#change-the-dimensions-of-images-in-markdown

Even though GitHub doesn't do it by default, we could do it almost for free (all that's required to add is a simple regex matcher on the text literal appended to the native image). So I'd put this PR on hold until this functionality is supported.