Fix the code block handling for our custom Markdown syntax

There were some cases that was hit that revealed some mistakes in the
code block checking code, which is now fixed. Basically, we just needed
to make sure the indices were updated at the right times. I also took
some time to clean up some of the comments, and magic numbers used here.

A new test case was added that was failing before in real-world testing.

autotests/texthandlertest.cpp

@@ -264,6 +264,8 @@ void TextHandlerTest::sendCustomTags_data()
     QTest::newRow("inside code block spoiler") << u"```||apple||```"_s << u"<code>||apple||</code>"_s;
     QTest::newRow("outside code block spoiler") << u"||apple|| ```||banana||``` ||pear||"_s
                                                 << u"<span data-mx-spoiler>apple</span> <code>||banana||</code> <span data-mx-spoiler>pear</span>"_s;
+    QTest::newRow("complex spoiler") << u"Between `formFactor == Horizontal||Vertical` and `location == top||left||bottom||right`"_s
+                                     << u"Between <code>formFactor == Horizontal||Vertical</code> and <code>location == top||left||bottom||right</code>"_s;
 
     // strikethrough
     QTest::newRow("incomplete strikethrough") << u"~~test"_s << u"~~test"_s;