feat: use grafana-toolkit to build a plugin

- fix linter warnings
- use yarn: remove Gruntfile.js and package-lock.json

++

dist/libs/d3-scale-chromatic/colors.js

@@ -1,23 +1,5 @@
-"use strict";
-
-System.register([], function (_export, _context) {
-  "use strict";
-
-  _export("default", function (specifier) {
-    var n = specifier.length / 6 | 0,
-        colors = new Array(n),
-        i = 0;
-
-    while (i < n) {
-      colors[i] = "#" + specifier.slice(i * 6, ++i * 6);
-    }
-
-    return colors;
-  });
-
-  return {
-    setters: [],
-    execute: function () {}
-  };
-});
-//# sourceMappingURL=colors.js.map
+export default function(specifier) {
+  var n = specifier.length / 6 | 0, colors = new Array(n), i = 0;
+  while (i < n) colors[i] = "#" + specifier.slice(i * 6, ++i * 6);
+  return colors;
+}