feat: use grafana-toolkit to build a plugin

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

++

.editorconfig

@@ -0,0 +1,16 @@
+# http://editorconfig.org
+root = true
+
+[*]
+indent_style = space
+indent_size = 2
+charset = utf-8
+trim_trailing_whitespace = true
+insert_final_newline = true
+max_line_length = 120
+
+[*.{js,ts,tsx,scss}]
+quote_type = single
+
+[*.md]
+trim_trailing_whitespace = false