A variable name should not start with `$`

[saig0]

Describe the bug Currently, a variable name can start with a $ sign. According to the spec, $ is not a valid start of a variable name.

Related to https://github.com/bpmn-io/feel-editor/issues/28.

To Reproduce Steps to reproduce the behavior:

1. Evaluate the expression $a with a context {"$a": 1}
2. Verify that the expression is evaluated successfully

Expected behavior The $ sign should not be a valid start of a variable name. As a result, an expression should not evaluate successfully (i.e. should not parse successfully) if it contains a variable name that starts with a $ sign.

Environment

• FEEL engine version: 1.15
• Affects:
  • Camunda Automation Platform 7: [7.x]
  • Zeebe broker: [0.x]