Centralise indented code block checks

In markdown-it-py (https://github.com/executablebooks/markdown-it-py/commit/9251695727cfa948bb18fc76a5dc85495cacc361) and for its plugins (https://github.com/executablebooks/mdit-py-plugins/commit/85f7ed312c530bb7faf98c079c71379122082556 + https://github.com/executablebooks/mdit-py-plugins/commit/9e5aff87a598504c46f9488a0d10423c698de9fa) I have centralised the check - present in all block rules - for a block indented by greater than 4 spaces, i.e. the check for an indented code block, and deactivate it when the code rule is disabled.

This allows for behaviour similar to e.g. https://github.com/jgm/djot and https://mdxjs.com/, whereby you can indent syntax blocks arbitrarily, e.g.

<div>
    <div>
        
        > arbitrarily indented syntax block

    </div>
</div>

at the moment, in the code here, IMO it's a bit unintuitive that most block rules will not work like this, even if you disable code blocks