[BUG] [Dart] Dollar sign in parameter names generated invalid code

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue? swagger.json
• Have you validated the input using an OpenAPI validator (example)? Yes, it's valid.
• Have you tested with the latest master to confirm the issue still exists?
• Have you searched for related issues/PRs? Yes, no related issues for dart exist.
• What's the actual output vs expected output? Incorrect string literals get generated:
  
• [Optional] Sponsorship to speed up the bug fix or feature request (example)

Description

Parameters with a dollar sign ($) in their names cause a syntax error in generated code. This is because a dollar sign is used for string interpolation in dart.

openapi-generator version

5.1.1

OpenAPI declaration file content or url

The following swagger.json contains an operation with a couple of parameters with a $ prefix, such as $top and $search.

Generation Details

Dart language/generator. I think it also applies to other dart variants. Default config.

Steps to reproduce

Generate a dart client with the openapi spec linked above. Config specifics seems to be irrelevant for this issue. Notice how there's a couple of syntax errors for any parameter starting with a $

Related issues/PRs

None

Suggest a fix

Make sure dollar signs are escaped in string literals

Start with the linked swagger.json and generate a Dart client using the Dart language/generator with the default configuration. Inspect the generated code for parameters such as $top and $search, then verify that dollar signs are escaped in string literals and the generated client no longer has syntax errors.