Highlight Nim default in Markdown code in .nim (#20110)

Highlight Nim by default in Markdown code in .nim

tests/stdlib/trst.nim

@@ -474,7 +474,12 @@ suite "RST parsing":
   let expectCodeBlock = dedent"""
       rnCodeBlock
         [nil]
-        [nil]
+        rnFieldList
+          rnField
+            rnFieldName
+              rnLeaf  'default-language'
+            rnFieldBody
+              rnLeaf  'Nim'
         rnLiteralBlock
           rnLeaf  '
       let a = 1

tests/stdlib/trstgen.nim

@@ -582,9 +582,14 @@ Test literal block
 ```
 let x = 1
 ``` """
-    let output1 = input1.toHtml
+    let output1 = input1.toHtml({roSupportMarkdown, roPreferMarkdown})
     doAssert "<pre" in output1 and "class=\"Keyword\"" notin output1
 
+    # Check Nim highlighting by default in .nim files:
+    let output1nim = input1.toHtml({roSupportMarkdown, roPreferMarkdown,
+                                    roNimFile})
+    doAssert "<pre" in output1nim and "class=\"Keyword\"" in output1nim
+
     let input2 = """
 Parse the block with language specifier:
 ```Nim