Use lark on Python 3.8+ while keeping lark-parser for older Pythons

[kendonB]

Motivation

• The project referenced the legacy lark-parser package which prevents using the renamed lark package on modern Python installations and can cause dependency conflicts (see issue Update lark version? #384).
• The repository still advertises support for older Python versions, so a migration must not break legacy interpreter compatibility.

Description

• Update install_requires in setup.py to conditionally install lark>=1.0.0 for python_version>='3.8' and keep lark-parser>=0.8.0 for python_version<'3.8' to preserve backward compatibility.\n- Replace lark-parser with lark in documentation/requirements.txt to reflect modern documentation build environments.\n- Add an Unreleased note to CHANGELOG.rst documenting the dependency split and rationale.\n\n### Testing\n- Ran python -m py_compile setup.py to validate the modified setup.py, and the compilation succeeded.\n\n------\nCodex Task

[drmfinlay]

Thank you, Kendon, for the pull requests and comments you've made on this repo lately. The motivation/description and testing stuff above is unneeded. The changes speak for themselves.