[2.0.5] Copy variant templates when converting patterns to components

Problem/Motivation

When converting patterns to components using the @ui-patterns:migrate@ Drush command, we are notified of the existence of variant templates in our pattern with a message telling that variants doesn't exist in SDC so the template has not been migrated.

Steps to reproduce

1. Create a pattern with variants
2. Implement a variant template (ie. @pattern-ID--variant-VID.html.twig@)
3. Run the migration command

Proposed resolution

Even if variant templates are not supported with SDC, that doesn't mean we cannot use variants to make variations on our markup. It's just a bit uglier because it relies on @if@ statements.
eg:

{% if variant == "my-variant" %}
markup here
{% elif variant == "my-other-variant" %}
other markup here
{% else %}
default markup here
{% endif %}

We can tune the command to output that kind of file.