Dart: use lower camel case for values

[Cantasura]

Related to this issue @jaumard

PR checklist

• [x] Read the contribution guidelines.
• [x] Pull Request title clearly describes the work in the pull request and Pull Request description provides details about how to validate the work. Missing information here may result in delayed response from the community.
• [x] Run the following to build the project and update samples:

  ./mvnw clean package || exit
  ./bin/generate-samples.sh ./bin/configs/*.yaml || exit
  ./bin/utils/export_docs_generators.sh || exit
  

  (For Windows users, please run the script in Git BASH) Commit all changed files. This is important, as CI jobs will verify all generator outputs of your HEAD commit as it would merge with master. These must match the expectations made by your contribution. You may regenerate an individual generator by passing the relevant config(s) as an argument to the script, for example ./bin/generate-samples.sh bin/configs/java*. IMPORTANT: Do NOT purge/delete any folders/files (e.g. tests) when regenerating the samples as manually written tests may be removed.
• [ ] File the PR against the correct branch: master (upcoming 7.x.0 minor release - breaking changes with fallbacks), 8.0.x (breaking changes without fallbacks)
• [x] If your PR is targeting a particular programming language, @mention the technical committee members, so they are more likely to review the pull request.

[wing328]

is this similar to the enumPropertyNaming option in java client generator?

ref: https://openapi-generator.tech/docs/generators/java/

[Cantasura]

It looks like that it could be same. I'm not sure though if the toVarName function of the AbstractCodegen only triggers for enum cases.

[wing328]

I'm not sure though if the toVarName function of the AbstractCodegen only triggers for enum cases.

i think it's used by all variable naming

[wing328]

cc @jaumard (2018/09) @josh-burton (2019/12) @amondnet (2019/12) @sbu-WBT (2020/12) @kuhnroyal (2020/12) @agilob (2020/12) @ahmednfwela (2021/08)

[kuhnroyal]

Nice new setting! But something seems wrong if there are changes to variable names in existing samples.

[wing328]

here is another PR related to Dart enum naming: https://github.com/OpenAPITools/openapi-generator/pull/20911

cc @epou