Initial implementation of the code block feature

[oskarwrobel]

Suggested merge commit message (convention)

Feature: Initial implementation of the code block feature. Closes ckeditor/ckeditor5#436. Closes ckeditor/ckeditor5#5664. Closes ckeditor/ckeditor5#5666.

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Requires ckeditor/ckeditor5-theme-lark#251.
To be followed by ckeditor/ckeditor5-autoformat#78.

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You can check out the documentation under development for more information (feature guide) https://github.com/ckeditor/ckeditor5-code-block/compare/i/5668?expand=1.

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TODO:

• Introduce Docs for the Code Block feature
• Configure CI for the code block repository
• Improve pasting to the code block
• How to move the selection under the code block when it is the last element in the document
• Enable auto formatting for code block feature

[dkonopka]

I've polished icon a bit:

Before:

After:

[jodator]

This is one huge of a PR :)

I was fixing smaller things I found just to speed up things and I the feedback is mostly to @oleq's additions.

From the bigger things to change IMO:

• split indent command (smaller == easier to digest === simpler)
• maybe the config should be a bit different

From the other things (follow ups?):

the dropdown menu has two scrolls:

the code block inside table is hidden under the code block lang label (make it transluscent? but this is definetely a follow up

[oleq]

the dropdown menu has two scrolls:

Fixed.

the code block inside table is hidden under the code block lang label (make it transluscent? but this is definetely a follow up

I went from min-width: 60px to 200px which helps (sort of). Such narrow code blocks do not make sense in the first place so I wouldn't worry about it anyway.

[jodator]

🚀 😍 LGTM merging after CI :D