feat[next]: config improvements

[DropD]

Description

• move 'next.errors.excepthook._verbose_exceptionstonext.config.VERBOSE_EXCEPTIONS`
• merge environment flag reading mechanisms into next.config.env_flag_to_bool
• test next.config.env_flag_to_bool
• completely move cache._persistent_cache_dir_path to next.config.BUILD_CACHE_PATH
• add a force_temporaries trait to GTFNBackendFactory
• test the force_temporaries trait of GTFNBackendFactory

Requirements

• All fixes and/or new features come with corresponding tests.
• Important design decisions have been documented in the approriate ADR inside the docs/development/ADRs/ folder.

If this PR contains code authored by new contributors please make sure:

• All the authors are covered by a valid contributor assignment agreement provided to ETH Zurich and signed by the employer if needed.
• The PR contains an updated version of the AUTHORS.md file adding the names of all the new contributors.

[tehrengruber]

Just two small things.