Introduce `--next` flag on `cz version --project`

[woile]

Description

returns the next version based on the project's config.

It could potentially support forcing the increment type. Example: --next=MAJOR or --next=PATCH

[bearomorphism]

Cross-checked against master (v4.15.1) by an claude-opus-4.7 validation pass while triaging #1976.

Status: partially fixed.

Form	Status
cz version --project --next MAJOR|MINOR|PATCH	implemented (commitizen/cli.py:577-587, commitizen/commands/version.py:93-103); covered by tests/commands/test_version_command.py::test_next_version
cz version --project --next (no value, derive increment from commit history)	not implemented — explicit TODO at commitizen/commands/version.py:85-91 returns "--next USE_GIT_COMMITS is not implemented yet.", locked in by tests/commands/test_version_command.py:289-292

The default --next form (no value) is the headline behaviour the issue asks for, so this issue should stay open until the bump-increment derivation in commitizen/commands/bump.py is refactored so version.py can call it and replace the USE_GIT_COMMITS TODO.

(Surfaced via the round-3 triage in #1976.)