fix: add missing diagram-js stylesheet to examples

[misiekhardcore]

related to bpmn-io/bpmn-js#2135

Proposed Changes

There were missing diagram-js.css imports missing in some of the examples, which caused for example no indication of selected items and in case of multi-selection - the selected elements were covered by a black rectangle (see the related issue for a GIF)

To try out the solution, simply open any example which and see that selected elements are properly indicated

Checklist

To ensure you provided everything we need to look at your PR:

• Brief textual description of the changes present
• Visual demo attached
• Steps to try out present, i.e. using the @bpmn-io/sr tool
• Related issue linked via Closes {LINK_TO_ISSUE} or Related to {LINK_TO_ISSUE}

[nikku]

As we discussed we'll not go this route, right @misiekhardcore? Instead we want to fix the issue at the core, ensure that the viewer does not ship with anything that requires external styles.

[misiekhardcore]

Yes, instead of doing this, we go for solving the root cause. This PR was opened to do so