Code blocks sometimes don't render first character

[JimKnoxx]

Version(s) affected

2.3, 2.4

Description

Hey there
We switched our markdown interpreter to commonmark. While reviewing old texts with the new markdown interpreter I noticed a strange behavior. In a special scenario with code blocks, tabs and lists, the resulting markdown 'forgets' the first character of a given code block.
I can reproduce it in a commonmark only project and will give you the code example below.

How to reproduce

I had a clean folder and only run composer require league/commonmark.
I created a index.php and filled it with following code:

<?php

require __DIR__ . '/vendor/autoload.php';

use League\CommonMark\CommonMarkConverter;

$converter = new CommonMarkConverter();

$str = <<<EOD
# Stuff

- Something more:
	```
	What's happening?
	```
EOD;

echo $converter->convert($str);

I ran php -S localhost:8000 -t .

The resulting output is:

[colinodell]

This type of issue tends to happen when tabs are used for indentation and we forget to advance the cursor by column (instead of by character). This should be a straightforward fix once we identify exactly where this is happening.