[dart] Improve types & imports

• don't use importMapping as it is intended for something different that is not possible in dart
• introduce imports map for dart specific features
• always import dart:core
• get rid of additionalReservedWords
• fix --type-mappings not working
• use required type mappings in samples
• no longer define additional reserved words as it is impossible to list all anyways, they can now be configured via type-mapping parameter
• improve dart-dio imports

Fixes #9029 (closed)

I think this is a much better solution and allows customization via CLI or config parameters. importMapping is meant to be used to map a class/type to that of a specific package, like List: java.util.List this is not possible in Dart. importMapping is being used internally in several places and if it is miss-used in the Dart generator as it was until now, it break the possibility of specifying typeMappings via CLI/config.

PR checklist

• Read the contribution guidelines.
• 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.
• Run the following to build the project and update samples:

  ./mvnw clean package 
  ./bin/generate-samples.sh
  ./bin/utils/export_docs_generators.sh

  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*. For Windows users, please run the script in Git BASH.
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CC @swipesight (2018/09) @jaumard (2018/09) @josh-burton (2019/12) @amondnet (2019/12) @sbu-WBT (2020/12) @kuhnroyal (2020/12) @agilob (2020/12)