PIPE-3211: define CASA aliases upfront and add export option for dependencies
- Move CASA environment shortcuts and aliases (`casa_pip`, `casa_python`)
upfront under Prerequisites in monolithic CASA documentation.
- Add `--export [FILE]` (`-o`) option to `scripts/install_dependencies.py`
to extract dependencies in requirements.txt format.
- Add TOML parser fallback (tomllib -> tomli -> pip._vendor.tomli) in
`scripts/install_dependencies.py` for Python < 3.11 (monolithic CASA) support.
- Document standalone Python 3.11+ tomllib one-liners with standard line wrapping.
- Clarify test execution commands in `daskhelpers_test.py` docstring.
- Cross-reference monolithic CASA environment setup from pytest reference guide.
