Polymer 3 compatibility

Changes Since v1.x:

• e01704c Add empty dependencies section for P3 conversion

• e61f778 keep window.Vaadin initiation in the P3 version

  add a guard against polymer-modulizer removing window.Vaadin initializer

• 39eb58d exclude build, .vscode and yarn.lock from the npm package

• b95785b remove polymer from bower deps

• f9def48 load dev-mode code from source code comments (removed by minification)

  • change the dev mode detection: remove 'is bundled' check, add 'is minified' check
  • remove dev mode code loading from HTML imports / ES modules / etc
  • add dev mode code loading from source code comments