Add leading and trailing icon link decorations

[bjdodson-openai]

Summary

Add inline icon support to LinkDecoration (Painter + Composable) so links can render leading/trailing icons inline with text while preserving line‑wrap and underline behavior.
Updates the Markdown demo to showcase the new capability.

References: follow‑up to LinkDecoration groundwork in #49

Changes

• LinkDecoration now accepts inlineContent via LinkInlineContent (leading/trailing + spacing + hit target).
• New InlineIconSpec sealed type with:
  • Painter (fast path for icons)
  • Composable (escape hatch for custom content)
• New LinkContext and LinkComposableContent fun interface for composable content.
• RichTextString now resolves link decorations into ranges that include link text.
• Text builds a decorated AnnotatedString with inline placeholders and adjusts underline ranges accordingly.
• Markdown demo shows a trailing arrow icon for dotted links.

Behavior

• Icons are inserted inline as placeholders, so they wrap naturally.
• Underline drawing still respects line wrapping and excludes icons (underline stays on link text).
• Optional includeInHitTarget expands link tap area to include the icons.

RTL / Layout

• Inline icons follow logical leading/trailing order, so they appear before/after the text in RTL appropriately.
• Spacing uses start/end padding to respect layout direction.

Testing

Notes

• LinkDecoration.matcher already supports (destination, text) so icon insertion can be context‑aware.
• This is a middle‑ground API: efficient for common icon use cases while still allowing custom composables.

Codex

Prompt/context: “Add leading/trailing icon support to LinkDecoration using a middle‑ground API (Painter + Composable), integrate inline placeholders into Text so icons wrap with link
text, keep underline drawing correct, and update the Markdown demo with an example.”