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/ramp.js

@@ -1,19 +1,5 @@
-"use strict";
+import d3 from 'd3';
 
-System.register(["d3"], function (_export, _context) {
-  "use strict";
-
-  var d3;
-
-  _export("default", function (scheme) {
-    return d3.interpolateRgbBasis(scheme[scheme.length - 1]);
-  });
-
-  return {
-    setters: [function (_d) {
-      d3 = _d.default;
-    }],
-    execute: function () {}
-  };
-});
-//# sourceMappingURL=ramp.js.map
+export default function(scheme) {
+  return d3.interpolateRgbBasis(scheme[scheme.length - 1]);
+}