fix rendering of code blocks escaped with multiple backticks or containing backslashes

#KT-12998 Fixed
 #KT-12999 Fixed

idea/testData/editor/quickDoc/OnMethodUsageWithMarkdown.kt

@@ -12,6 +12,9 @@
  * Single stars embedded in a word like Embedded*Star have to be preserved as well.
  *
  * Exclamation marks are also important! Also in `code blocks!`
+ *
+ * bt+ : ``prefix ` postfix``
+ * backslash: `\`
  */
 fun testMethod() {
 
@@ -30,3 +33,4 @@ fun test() {
 //INFO: <p><a href="psi_element://C">See <strong>this</strong> class</a></p>
 //INFO: <p>This is <em>emphasized text</em> but text_with_underscores has to preserve the underscores. Single stars embedded in a word like Embedded*Star have to be preserved as well.</p>
 //INFO: <p>Exclamation marks are also important! Also in <code>code blocks!</code></p>
+//INFO: <p>bt+ : <code>prefix ` postfix</code> backslash: <code>\</code></p>