Correctly handle *ast.AutoLink nodes in the plainmarkdown function (

#361)

* Write the node URL for `AutoLink` nodes

* Add changelog entry

.changes/unreleased/BUG FIXES-20240419-154226.yaml

@@ -0,0 +1,6 @@
+kind: BUG FIXES
+body: 'generate: fixed a bug where the `plainmarkdown` function did not output plain
+  URLs'
+time: 2024-04-19T15:42:26.250348-04:00
+custom:
+  Issue: "361"

internal/mdplain/renderer.go

@@ -61,9 +61,12 @@ func (r *TextRender) Render(w io.Writer, source []byte, n ast.Node) error {
 				return ast.WalkSkipChildren, nil
 			}
 			return ast.WalkContinue, nil
-		case *ast.AutoLink, *extAST.Strikethrough:
+		case *extAST.Strikethrough:
 			out.Write(node.Text(source))
 			return ast.WalkContinue, nil
+		case *ast.AutoLink:
+			out.Write(node.URL(source))
+			return ast.WalkSkipChildren, nil
 		case *ast.CodeSpan:
 			out.Write(node.Text(source))
 			return ast.WalkSkipChildren, nil

internal/mdplain/testdata/markdown.md

@@ -50,6 +50,7 @@ These are the elements outlined in John Gruber’s original design document. All
 
 [Relative Link](#Code)
 
+Plain URL: https://www.markdownguide.org
 
 ### Image
 

internal/mdplain/testdata/mdplain.txt

@@ -23,6 +23,7 @@ Horizontal Rule
 Link
 Markdown Guide https://www.markdownguide.org
 Relative Link
+Plain URL: https://www.markdownguide.org
 Image
 
 Extended Syntax