Add data structures for chart plugin system (#6028)

* add unit tests

* add test structure

* add unit tests for Registry

* add LoaderRegistry unit test

* add unit test for makeSingleton

* add type check

* add plugin data structures

* simplify API

* add preset tests

* update test message

* fix lint

* update makeSingleton

* update Plugin, Preset and unit test

* revise Registry code

* update unit test, add remove function

* update test

* update unit test

* update plugin unit test

* add .keys(), .entries() and .entriesAsPromise()

* update test description

superset/assets/spec/javascripts/visualizations/models/Plugin_spec.js

@@ -0,0 +1,48 @@
+import { describe, it } from 'mocha';
+import { expect } from 'chai';
+import Plugin from '../../../../src/visualizations/core/models/Plugin';
+
+describe('Plugin', () => {
+  it('exists', () => {
+    expect(Plugin).to.not.equal(undefined);
+  });
+
+  describe('new Plugin()', () => {
+    it('creates a new plugin', () => {
+      const plugin = new Plugin();
+      expect(plugin).to.be.instanceof(Plugin);
+    });
+  });
+
+  describe('.configure(config, replace)', () => {
+    it('extends the default config with given config when replace is not set or false', () => {
+      const plugin = new Plugin();
+      plugin.configure({ key: 'abc', foo: 'bar' });
+      plugin.configure({ key: 'def' });
+      expect(plugin.config).to.deep.equal({ key: 'def', foo: 'bar' });
+    });
+    it('replaces the default config with given config when replace is true', () => {
+      const plugin = new Plugin();
+      plugin.configure({ key: 'abc', foo: 'bar' });
+      plugin.configure({ key: 'def' }, true);
+      expect(plugin.config).to.deep.equal({ key: 'def' });
+    });
+    it('returns the plugin itself', () => {
+      const plugin = new Plugin();
+      expect(plugin.configure({ key: 'abc' })).to.equal(plugin);
+    });
+  });
+
+  describe('.resetConfig()', () => {
+    it('resets config back to default', () => {
+      const plugin = new Plugin();
+      plugin.configure({ key: 'abc', foo: 'bar' });
+      plugin.resetConfig();
+      expect(plugin.config).to.deep.equal({});
+    });
+    it('returns the plugin itself', () => {
+      const plugin = new Plugin();
+      expect(plugin.resetConfig()).to.equal(plugin);
+    });
+  });
+});