Integrate `first-item` FEEL validation gracefully, as a warning

[nikku]

Proposed Changes

This ensures that first-item is not indicated as a hard error, but a warning. It also prevents first-item matches, along with any non-syntax errors, from showing off as a Unparseable FEEL expression syntax error.

Try out via

npx @bpmn-io/sr bpmn-io/bpmn-js-properties-panel -l bpmn-io/properties-panel#first-item-warning

Related to https://github.com/bpmn-io/bpmn-js-properties-panel/issues/1078

Checklist

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

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