Introduced to-do list styles

[dkonopka]

Suggested merge commit message (convention)

Other: Introduced to-do list styles (moved from ckeditor5-list repository). Closes https://github.com/ckeditor/ckeditor5-list/issues/146. Closes https://github.com/ckeditor/ckeditor5-list/issues/136

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Additional information

1. Fixed selecting "unchecked" list element on Safari
2. Fixed vertical align of checkmark with larger fonts
3. Fixed WCAG contrast of checkmark (now passing 3.0)

[oleq]

Looks like using relative values

	left: calc( var(--ck-spacing-large) * -1.5 );
	margin-right: calc( var(--ck-spacing-large) * -1 );

wasn't the best idea and we'll need to bring back the absolute values

This is because when the font size is applied it wraps both the <label> and the text

[oleq]

This should be reverted when 7e7d268 goes into master.

[dkonopka]

Ok, now it's work properly with various size of font and checkmark size:

And RTL (thanks for adding it ❤️, I completely forgot about this!):