Represent code macros, verbatim and monospace with distinct syntaxes in CommonMark Markdown 1.2

The code macro, verbatim and monospace are all rendered as Markdown code spans or code blocks, and the Markdown parser reads code spans and code blocks back as code macros. Thus verbatim and monospace content becomes a code macro after a round trip (and the formatting located inside monospace is lost), and a code macro without language gets a language="none" parameter.

The idea is to use a distinct Markdown syntax (raw HTML being allowed in CommonMark) for each of them, so that they can be rendered to Markdown and parsed back without loss:

{{code ...}}...{{/code}}

##x##

<code>x</code>

<pre>...</pre>

<tt>...</tt>

Notes:

• A code span or a fenced code block without info string keeps being read as a code macro with language="none", since Markdown content isn't highlighted unless a language is specified (see MARKDOWN-65).
• Only <code>, <tt> and <pre> without attributes would be recognized, the other HTML keeps being read as HTML macros. Hand written <code>, <tt> and <pre> located in existing Markdown pages would be read as monospace and verbatim instead of HTML macros (which renders almost the same).
• Mapping <code> to monospace is aligned with XRENDERING-780. Using a code macro for Markdown code is a temporary solution until XRENDERING-266 is implemented.

Forum proposal: (link to be added)