refactor: create files for each theme

[sehnryr]

Proposed changes

+ lib/src/cookie_jar for CookieJar
+ lib/src/exception.dart for every custom Exceptions
+ lib/src/storage.dart for the CookieJars storage

- lib/src/response_error.dart

Type of changes

What types of changes does your code introduce?

Put an x in the boxes that apply.

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist

• I have read the CONTRIBUTING doc
• My code follows the Dart coding convention
• Lint and unit tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works.
• I have commented my code, particularly in hard-to-understand areas
• I have added necessary documentation (if appropriate)

Further comments