Pattern Lab only loads the pattern engines from the config

[ringods]

Closes #1251

Summary of changes:

• Load the engines which are configured in the config first
• Fallback to the old way of scanning node_modules if no engines are configured
• Documentation added on the engines property in the Pattern Lab config file. URLs in the source code should point to this property once it is published during the release.
• Proper deprecation messages should be in place.

[coveralls]

Coverage decreased (-1.8%) to 75.0% when pulling 5d376a5 on feature/resolver-pattern-engines into 1d28ab0 on dev.

[JosefBredereck]

I can not approve that PR at the moment. The preview is not building.

Pattern Engine undefined: good to go
Found plugin: @pattern-lab/plugin-tab
Attempting to load and initialize plugin.
where is the engine?
Error rendering pattern lab header
Omitting atoms-colors from styleguide patterns because it has an underscore prefix.
where is the engine?
error during header render()
where is the engine?
error during footer render()
where is the engine?
Error building buildFooterHTML

[JosefBredereck]

When I add engine mustache, it is working. (We need to get rid of that engine to be required everywhere but it's part of the UI-kit) But it still messages:
Pattern Engine undefined: good to go
Pattern Engine undefined: good to go

[...]
    "mustache": {
      "package": "@pattern-lab/engine-mustache",
      "fileExtensions": [
        "mustache"
      ]
    }
[...]

[stale]

It's hard to keep track of everything. This issue has been automatically marked as stale because it has not had recent activity, neither from the team nor the community. It will be closed if no further activity occurs. Please consider adding additional info, volunteering to contribute a fix for this issue, or making a further case that this is important to you, the team, and the project as a whole. Thanks!

[mfranzke]

@ringods first of all thanks a lot for your work that you did so far regarding this topic. I tried to follow up on the feedback by @JosefBredereck and checked in some minor changes of how I would think about solving this topic. I hope that was in your interest.

I've integrated some workarounds, that we should be able to remove with #1455; as even also mentioned by @JosefBredereck we should get rid of mustache in favour of its successor handlebars anyhow.

[JosefBredereck]

Interesting approach. Do we need to change this to handlebars after you changed the package with #1456?

[mfranzke]

It's mainly a mitigation, because we still need the mustache engine as we reference mustache templates all over the place, and without it we would get errors like that the header isn't available. By #1455 we should be able to remove this code part again.