Ensure existing values are preserved

[nikku]

Context

• With https://github.com/bpmn-io/bpmn-js-element-templates/pull/82 we removed the application of filtered element templates, resolving an infinite loop.
• We broke a core guarantee (valid values are kept) but did not notice due to missing test coverage

This PR

• Restores the pre https://github.com/bpmn-io/bpmn-js-element-templates/pull/82 behavior and adds relevant test coverage
• It ensures that a template is always conditionally applied
• It ensures that condition is only applied once per update cycle, from the original template
• This way we allow the evaluation to stabilize, and stop evaluating recursively (eventually)

---

Related to https://github.com/camunda/camunda-modeler/issues/4249

[nikku]

I'll conduct an E2E test on this feature tomorrow, using camunda/connectors as a foundation.

[nikku]

Pending: Integration test against current connector templates. I'm still trying go get these from somewhere.

[nikku]

Thanks @marstamm. Squashed https://github.com/bpmn-io/bpmn-js-element-templates/pull/89 into this PR.

[nikku]

During integration testing I found an interesting follow-up issue: https://github.com/camunda/camunda-modeler/issues/4252.