Add MarkdownRenderer abstraction to v0.9 web_core

Currently, v0.9 renderers use MarkdownRenderer from v0.8 in web_core. This should not be the case. Let's export a duplicate version from v0.9 web_core, to keep the codebases separate. I believe the markdown-it package should implement *both* interfaces.

Find the v0.8 MarkdownRenderer in web_core and understand its interface. Create a duplicate version in the v0.9 web_core module, ensuring the markdown-it package implements both interfaces. Check existing renderer usage to ensure compatibility. Verify the separation by testing that v0.9 code no longer depends on the v0.8 version.

索引モデルが issue の本文から書いたものです。