Litrepl

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Minor environment changes

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1. **Python**
- Code refactoring for better code management and efficiency: moved main
logic in `litrepl` from `python/bin/litrepl` to a new file
[python/litrepl/main.py](./python/litrepl/main.py).
- Improved handling of subprocess management for the Python and AI
interpreters, ensuring proper cleanup and output management. Additionally,
calls to `os.system` were mostly replaced with native Python calls.
- Added robust error handling and message retrieval for the interpreter
interface, providing clearer errors when the interpreter fails.

2. **Vim**
- Introduced the default environment variable check for `LITREPL_WORKDIR`,
`LITREPL_PYTHON_AUXDIR`, `LITREPL_AI_AUXDIR` etc. environment variables.
- Updated Vim functions to consider both buffer and global settings,
improving flexibility in command generation (`LitReplGet` helper function).
- Updated commands related to AI tasks like `:LAIStyle`, `:LAIFile`, and
`:LAICode` to handle rephrasing and AI interaction more efficiently.
- Adjusted internal Vim functions to handle text width settings and cursor
positionings.

3. **Environment**
- Replaced `test.sh` with [runtests.sh](./sh/runtests.sh) in the build and test
process for enhanced test execution flow.
- Allowed more environment configuration by introducing more settings that
default from environment variables, streamlining setup customization.
- Updated Nix build script ([default.nix](./default.nix)) to reflect changes
in the test script location and hash updates for source verification.
- Added this changelog and the AI-powered changelog helper script
[diffchanges.sh](./sh/diffchanges.sh).

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1. **Python**
- Added assertions to ensure a successful attachment to the interpreter,
enhancing error handling robustness.

2. **Vim**
- Enhanced [vim/plugin/litrepl.vim](./vim/plugin/litrepl.vim) by preserving
caret positions and restructuring comments for function alignment.
- Significant expansions in
[vim/plugin/litrepl_extras.vim](./vim/plugin/litrepl_extras.vim) with
multiple new functions for region handling, AI task management, and
interaction across different scopes, substantially improving user
experience with AI functionalities in Vim. Several new commands such as
`LAITell`, `LAICont`, `LAIStyle`, `LAIFile`, and `LAICode` are introduced
to support a wide range of code and AI-related tasks.

3. **Environment**
- Added a new markdown file `./doc/screencast.md` with examples for
evaluating Python code in Vim, offering insights into the screencast
documentation process.
- Adjusted `screencast.sh` to be executable and made improvements to
accommodate a target file parameter.
- Introduced a new test function `test_vim_ai_query` in `test.sh`,
facilitating new test coverage for AI query functionality.