fix: mark feel: required that does not start with = as deprecated

[jarekdanielak]

Related to camunda/camunda-modeler#4967

Proposed Changes

In a feel: required property, mark value as deprecated if it doesn't start with =.

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]

Verified this is indicated in Sublime Text:

I could not verify this doesn't trigger an error, nor could I verify that we can distinguish errors and deprecation in our tooling.

[nikku]

As we start to use deprecated more it probably makes sense to add respective test coverage:

• What is deprecated
• How can we source the deprecated information for tooling (problems panel)

[barmac]

Regarding deprecated usage, some discovery was done in #187 (comment)

[nikku]

I don't think #187 (comment) is conclusive. In this gist I demonstrate how we can have both, deprecation warnings and errors when validating with AJV. Question is if we need it, or if hints in JSON-schema enabled editors is enough.