Markdown: MarkdownRenderer, MarkdownEditor, highlights and block ranges read with markdown-rs

The tree comes from markdown::to_mdast with GitHub's flavor and math.
References resolve against their definitions, an image splits its
paragraph, a one-line $$ formula stands as a block. BlockEditor's
import and export keep pulldown-cmark.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

Cargo.lock

@@ -1649,6 +1649,7 @@ dependencies = [
  "jiff",
  "js-sys",
  "log",
+ "markdown",
  "objc",
  "pulldown-cmark",
  "qrcode",
@@ -3611,6 +3612,15 @@ dependencies = [
  "libc",
 ]
 
+[[package]]
+name = "markdown"
+version = "1.0.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "a5cab8f2cadc416a82d2e783a1946388b31654d391d1c7d92cc1f03e295b1deb"
+dependencies = [
+ "unicode-id",
+]
+
 [[package]]
 name = "markup5ever"
 version = "0.39.0"
@@ -6576,6 +6586,12 @@ version = "0.4.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "ce61d488bcdc9bc8b5d1772c404828b17fc481c0a582b5581e95fb233aef503e"
 
+[[package]]
+name = "unicode-id"
+version = "0.3.7"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "65dd75ab2d9d379dbe86d4059e91d8b5a6e7054865eaba1ddbfc7a817ff96392"
+
 [[package]]
 name = "unicode-ident"
 version = "1.0.26"