Code blocks must be left intact

[fredck]

Describe the bug
Code blocks are very controlled content, where the author has intent (and full responsibility) on everything inside it. Right now, tabs inside code blocks are replaced by 4 spaces. This is (probably in most of the cases) semantically incorrect.

To Reproduce
Steps to reproduce the behavior:

1. Marked Demo
2. CommonMark Demo

Expected behavior
The contents of the code block should be left intact (tabs untouched).

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Many times, code in a document is used as an example or reference to be copied/pasted or used "as is". Marked is not aware of the purpose of the code, so it should not take action on it and leave it intact.

This is a real world example of this problem: ckeditor/github-writer#170. The code block is a suggestion for changes to source code and they can be merged as the author wrote it. Marked is changing the code though and, basically, breaking the author contribution.

I can imagine this being a problem on many other contexts as well.

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I've seen similar issues here for the same problem. Most of them are not clear and mainly closed. I wanted to refresh this problem with this issue.

[UziTech]

Similar to #1559

[azmy60]

It looks like this has been fixed since 4.0.14

[UziTech]

Not completely, lines in code blocks that start with tabs are still converted to spaces. demo