don't parse lines starting with @ as KDoc tags if they're inside a Markdown code fence

#KT-11789 Fixed

compiler/testData/kdoc/lexer/codeBlocks.kt

@@ -0,0 +1,14 @@
+/**
+ * @foo This is text.
+ * ``` kotlin
+ * @foo This is code block.
+ * ```
+ * @foo This is text.
+ * ~~~
+ * @foo This is code block
+ * With multiple lines.
+ * ~~~
+ * @foo This is yet more text.
+ * ```
+ * @foo This is an unclosed code block.
+ */

compiler/testData/kdoc/lexer/codeBlocks.txt

@@ -0,0 +1,48 @@
+KDOC_START ('/**')
+WHITE_SPACE ('\n ')
+KDOC_LEADING_ASTERISK ('*')
+KDOC_TEXT (' ')
+KDOC_TAG_NAME ('@foo')
+WHITE_SPACE (' ')
+KDOC_TEXT ('This is text.')
+WHITE_SPACE ('\n ')
+KDOC_LEADING_ASTERISK ('*')
+KDOC_TEXT (' ``` kotlin')
+WHITE_SPACE ('\n ')
+KDOC_LEADING_ASTERISK ('*')
+KDOC_TEXT (' @foo This is code block.')
+WHITE_SPACE ('\n ')
+KDOC_LEADING_ASTERISK ('*')
+KDOC_TEXT (' ```')
+WHITE_SPACE ('\n ')
+KDOC_LEADING_ASTERISK ('*')
+KDOC_TEXT (' ')
+KDOC_TAG_NAME ('@foo')
+WHITE_SPACE (' ')
+KDOC_TEXT ('This is text.')
+WHITE_SPACE ('\n ')
+KDOC_LEADING_ASTERISK ('*')
+KDOC_TEXT (' ~~~')
+WHITE_SPACE ('\n ')
+KDOC_LEADING_ASTERISK ('*')
+KDOC_TEXT (' @foo This is code block')
+WHITE_SPACE ('\n ')
+KDOC_LEADING_ASTERISK ('*')
+KDOC_TEXT (' With multiple lines.')
+WHITE_SPACE ('\n ')
+KDOC_LEADING_ASTERISK ('*')
+KDOC_TEXT (' ~~~')
+WHITE_SPACE ('\n ')
+KDOC_LEADING_ASTERISK ('*')
+KDOC_TEXT (' ')
+KDOC_TAG_NAME ('@foo')
+WHITE_SPACE (' ')
+KDOC_TEXT ('This is yet more text.')
+WHITE_SPACE ('\n ')
+KDOC_LEADING_ASTERISK ('*')
+KDOC_TEXT (' ```')
+WHITE_SPACE ('\n ')
+KDOC_LEADING_ASTERISK ('*')
+KDOC_TEXT (' @foo This is an unclosed code block.')
+WHITE_SPACE ('\n ')
+KDOC_END ('*/')