Switch to swift-markdown (#3)

Changes:

- Removes embedded `cmark` in favour of [`apple/swift-cmark`](https://github.com/apple/swift-cmark)
- Removes AST code in favour of [`apple/swift-markdown`](https://github.com/apple/swift-markdown)
- Removes DebugVisitor in favour of `document.debugDescription()` from `swift-markdown`
- Support tasklist/checkbox

Later:

- Restore ordered list starts, depends on: swiftlang/swift-markdown#22
- Restore various Down options, depends on: swiftlang/swift-markdown#23

Package.resolved

@@ -1,6 +1,24 @@
 {
   "object": {
     "pins": [
+      {
+        "package": "cmark-gfm",
+        "repositoryURL": "https://github.com/apple/swift-cmark.git",
+        "state": {
+          "branch": "gfm",
+          "revision": "fce0dee92a10ec75b65ef4e9ca1b341db4c6d2ea",
+          "version": null
+        }
+      },
+      {
+        "package": "swift-markdown",
+        "repositoryURL": "https://github.com/apple/swift-markdown.git",
+        "state": {
+          "branch": "main",
+          "revision": "caafc56d3794a08c2203fe417b3aff81e2ab2fc1",
+          "version": null
+        }
+      },
       {
         "package": "SnapshotTesting",
         "repositoryURL": "https://github.com/pointfreeco/swift-snapshot-testing.git",

Package.swift

@@ -16,27 +16,15 @@ let package = Package(
         )
     ],
     dependencies: [
-        .package(
-            url: "https://github.com/pointfreeco/swift-snapshot-testing.git",
-            from: "1.9.0"
-        ),
+        .package(url: "https://github.com/apple/swift-markdown.git", .branch("main")),
+        .package(url: "https://github.com/pointfreeco/swift-snapshot-testing.git", from: "1.9.0"),
     ],
     targets: [
-        .target(
-            name: "libcmark",
-            dependencies: [],
-            path: "Sources/cmark",
-            exclude: [
-                "include",
-                "case_fold_switch.inc",
-                "entities.inc",
-                "COPYING"
-            ],
-            publicHeadersPath: "./"
-        ),
         .target(
             name: "Down",
-            dependencies: ["libcmark"],
+            dependencies: [
+                .product(name: "Markdown", package: "swift-markdown")
+            ],
             path: "Sources/Down",
             exclude: ["Down.h"],
             resources: [

README.md

@@ -46,7 +46,7 @@ For a full description of features, see [`johnxnguyen/Down`](https://github.com/
 This fork swaps cmark for cmark-gfm, and supports the following extensions:
 
 - [x] Strikethrough
-- [ ] Task lists
+- [x] Task lists
 - [ ] Tables
 
 ### Supports