MDL-68496 grunt: Restructure grunt tasks into subdirectories

Prior to this change all Grunt features were in a single Gruntfile.js
but this has become difficult to manage and maintain.

This commit moves the existing dependencies for component calculation
and babel moduel definition into a new .grunt directory, and
restructures the existing tasks in Gruntfile.js into separate task
configuration files.

This improves the maintainability of the Grunt build system and allows
for easier future expansion.

.eslintrc

@@ -204,7 +204,7 @@
       }
     },
     {
-      files: ["**/amd/src/*.js", "**/amd/src/**/*.js", "Gruntfile*.js", "babel-plugin-add-module-to-define.js"],
+      files: ["**/amd/src/*.js", "**/amd/src/**/*.js", "Gruntfile.js", ".grunt/*.js", ".grunt/tasks/*.js"],
       // We support es6 now. Woot!
       env: {
         es6: true