Multi-line strings not covered by spec

[nikku]

Describe the bug

We support evaluating multi-line strings:

= "ABC
DEF"

This, however this is not covered by the DMN spec (\u000A is the line feed character):

To Reproduce

Execute above FEEL expression, see engine return ABC\nDEF.

Expected behavior

Syntax error (invalid FEEL syntax).

Environment

• FEEL engine version: 1.16.1
• Affects:
  • Camunda Automation Platform 7: ?
  • Zeebe broker: ?

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Downstream bug report in Camunda Modeler (FEEL engine and syntax highlighter not in sync) - https://github.com/camunda/camunda-modeler/issues/3859.