Bring the plugin API and the Gradle plugin from the legacy Tazel

Tazel Plugins stay, built with Gradle and published to Tazel Portal.
The api/ (com.progmasoft.tazel:tazel-api) and gradle-plugin/
(com.progmasoft.tazel.plugin) projects come from Progmasoft/tazel-legacy
with their tests, with these changes:

- tazelPlugin { } holds group, name and version. The descriptor
  META-INF/tazel-plugin.properties records them first, so the JAR says
  who the plugin is; before, ViGet recorded the publisher, name and
  version at push time. The group must be a reversed domain of at least
  two parts (Tazel Portal checks the namespace by the Sonatype rules),
  the name letters, digits and _ . -, and the version a Semantic
  Version. The version of the Gradle project is not the plugin's.
- The API version starts again at 1, so plugins built for the legacy
  Tazel (API 3) are not loaded.
- Documentation no longer speaks of ViGet, Tala targets or
  tazel.tala.run; the site is tazel.net.
- Dependency locking of the legacy build is not carried over, like in
  the rest of this build.

The root build includes both projects, so ./gradlew check runs the
Gradle TestKit tests of the plugin as well.

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