Don't require blank line before Markdown code (#20215)

Fixes bug reported in https://github.com/nim-lang/Nim/pull/20189
affecting nimforum.

tests/stdlib/trst.nim

@@ -576,6 +576,45 @@ suite "RST parsing":
       # | |
       # |  \ indentation of exactly two spaces before 'let a = 1'
 
+  test "no blank line is required before or after Markdown code block":
+    let inputBacktick = dedent"""
+        Some text
+        ```
+        CodeBlock()
+        ```
+        Other text"""
+    let inputTilde = dedent"""
+        Some text
+        ~~~~~~~~~
+        CodeBlock()
+        ~~~~~~~~~
+        Other text"""
+    let expected = dedent"""
+        rnInner
+          rnParagraph
+            rnLeaf  'Some'
+            rnLeaf  ' '
+            rnLeaf  'text'
+          rnParagraph
+            rnCodeBlock
+              [nil]
+              rnFieldList
+                rnField
+                  rnFieldName
+                    rnLeaf  'default-language'
+                  rnFieldBody
+                    rnLeaf  'Nim'
+              rnLiteralBlock
+                rnLeaf  '
+        CodeBlock()'
+            rnLeaf  ' '
+            rnLeaf  'Other'
+            rnLeaf  ' '
+            rnLeaf  'text'
+      """
+    check inputBacktick.toAst == expected
+    check inputTilde.toAst == expected
+
   test "option list has priority over definition list":
     check(dedent"""
         --defusages