BREAKING-CHANGE: Deprecate underscore prefix

[JosefBredereck]

Closes #1209
Closes #1182
Closes #1183

Summary of changes:

Change the way pattern lab is hiding patterns. Because all those file system hacks are not a good approach we decided to remove this functionality in favor of a more flexible way.

[JosefBredereck]

@bmuenzenmeyer @sghoweri @engelfrost @bradfrost @mfranzke @ringods
Is it more convenient to have the companion file for pattern-groups in the folder or next to the folder?
The same question appears for patter-sub-groups.

To overcome the pattern and group name matching I consider an underscore prefix for version 1 if it is a companion .md file of the group.

Examples (Sorted like the filesystem would do)

1. Inside the folder

_patterns/atoms/text/
_patterns/atoms/text/_text.md
_patterns/atoms/animation/
_patterns/atoms/animation/_animation.md
_patterns/atoms/_atoms.md
_patterns/molecules/blocks/
_patterns/molecules/blocks/_blocks.md
_patterns/molecules/cards/
_patterns/molecules/cards/_cards.md
_patterns/molecules/_molecules.md

2. Next to the folder

_patterns/atoms/text/
_patterns/atoms/animation/
_patterns/atoms/animation.md
_patterns/atoms/text.md
_patterns/molecules/blocks/
_patterns/molecules/cards/
_patterns/molecules/blocks.md
_patterns/molecules/cards.md
_patterns/atoms.md
_patterns/molecules.md

!! please upvote with 🚀 if you like version 1 and with 👀 if you prefer version 2 !!

EDIT: I know we have the top-level markdown files (as documentation patterns), which apply to version 2 in this example. The problem is, those are breaking the UI even without any change in the current version that no view all pages can be generated for folders with documentation blocks. Maybe we need to rethink how everything is working together.

EDIT 2: Based on all the information I got from issues and also the spec, I can now confirm, that version 2 was somehow implemented but, on the other hand, not. References: #415, the-spec/#28 and also an old PR where I gathered some more information. I will now proceed to use version 2 since it is already implemented in some way.

[coreylafferty]

If I understand this correctly, I'm on team 🚀. Generally I think all the files for a subfolder should be in that folder so it's self-contained. In our theme we have a script that automatically generates a subfolder containing the necessary files (e.g., twig, md, scss and yml files).

[JosefBredereck]

Keep in mind, this change is not for patterns. It is for groups and subgroups. But I'm totaly on you side. We also have scripts to create all required files in our company.

[JosefBredereck]

See my edits on my initial question, "commented on 18 Aug.". There are some hints for testing.