Markdown indented code blocks (#20473)

* Implement Markdown indented code blocks

Additional indentation of 4 spaces makes a block an "indented code block"
(monospaced text without syntax highlighting).
Also `::` RST syntax for code blocks is disabled.

So instead of
```rst
see::

  Some code
```

the code block should be written as
```markdown
see:

    Some code
```

* Migrate RST literal blocks :: to Markdown's ones

tests/stdlib/trst.nim

@@ -7,6 +7,8 @@ discard """
 
 [Suite] RST indentation
 
+[Suite] Markdown indentation
+
 [Suite] Warnings
 
 [Suite] RST include directive
@@ -124,12 +126,12 @@ suite "RST parsing":
     check(dedent"""
         Paragraph::
 
-        >x""".toAst == expected)
+        >x""".toAst(rstOptions = preferRst) == expected)
 
     check(dedent"""
         Paragraph::
 
-            >x""".toAst == expected)
+            >x""".toAst(rstOptions = preferRst) == expected)
 
   test "RST quoted literal blocks, :: at a separate line":
     let expected =
@@ -148,7 +150,7 @@ suite "RST parsing":
         ::
 
         >x
-        >>y""".toAst == expected)
+        >>y""".toAst(rstOptions = preferRst) == expected)
 
     check(dedent"""
         Paragraph
@@ -156,7 +158,7 @@ suite "RST parsing":
         ::
 
           >x
-          >>y""".toAst == expected)
+          >>y""".toAst(rstOptions = preferRst) == expected)
 
   test "Markdown quoted blocks":
     check(dedent"""
@@ -779,7 +781,7 @@ suite "RST parsing":
 
         code
 
-      """.toAst ==
+      """.toAst(rstOptions = preferRst) ==
       dedent"""
         rnInner
           rnLeaf  'Check'
@@ -788,6 +790,32 @@ suite "RST parsing":
             rnLeaf  'code'
       """)
 
+  test "Markdown indented code blocks":
+    check(dedent"""
+      See
+
+          some code""".toAst ==
+      dedent"""
+        rnInner
+          rnInner
+            rnLeaf  'See'
+          rnLiteralBlock
+            rnLeaf  'some code'
+      """)
+
+    # not a code block -- no blank line before:
+    check(dedent"""
+      See
+          some code""".toAst ==
+      dedent"""
+        rnInner
+          rnLeaf  'See'
+          rnLeaf  ' '
+          rnLeaf  'some'
+          rnLeaf  ' '
+          rnLeaf  'code'
+      """)
+
 suite "RST tables":
 
   test "formatting in tables works":
@@ -1238,6 +1266,31 @@ suite "RST indentation":
                 rnLeaf  'term3definition2'
       """)
 
+suite "Markdown indentation":
+  test "Markdown paragraph indentation":
+    # Additional spaces (<=3) of indentation does not break the paragraph.
+    # TODO: in 2nd case de-indentation causes paragraph to break, this is
+    # reasonable but does not seem to conform the Markdown spec.
+    check(dedent"""
+      Start1
+        stop1
+
+        Start2
+      stop2
+      """.toAst ==
+      dedent"""
+        rnInner
+          rnParagraph
+            rnLeaf  'Start1'
+            rnLeaf  ' '
+            rnLeaf  'stop1'
+          rnParagraph
+            rnLeaf  'Start2'
+          rnParagraph
+            rnLeaf  'stop2'
+            rnLeaf  ' '
+      """)
+
 suite "Warnings":
   test "warnings for broken footnotes/links/substitutions":
     let input = dedent"""

tests/stdlib/trstgen.nim

@@ -632,7 +632,7 @@ Test literal block
 ::
 
   check """
-    let output1 = input1.toHtml
+    let output1 = input1.toHtml(preferRst)
     doAssert "<pre>" in output1
 
   test "Markdown code block":