Use root node display property to set custom element root node display property

[matt-gadd]

Type: feature

The following has been addressed in the PR:

• There is a related issue
• All code has been formatted with prettier as per the readme code style guidelines
• Unit or Functional tests are included in the PR

Description:
Use the root node of the widgets display property and set it on the custom element node. If none set default to block which is more common/preferable than the custom element default of inline

This solution differs to what I originally postured here: #209 (comment), because I think this way is simpler, slightly more ergonomic and a lot of dojo/widgets already specify the desired display on their root node (for example: https://github.com/dojo/widgets/blob/master/src/theme/button.m.css#L5)

Resolves #209

Thanks to @jcfranco for raising the original issue.