Move REPL execution paths to main_modes/repl.py

[rolandwalker]

Description

Motivation: move code out of the monolithic main.py into logical layers.

There is no functional change for this refactor, just the creation of main_modes/repl.py and the migration of (some of) the REPL logic out of main.py.

There is much more to do, just in relation to the REPL. For example, complete_while_typing_filter() doesn't logically still belong in main.py, but it is bound up with state set in main.py. Likewise, the updates of the prompt string, and similar updates of window title and toolbar, logically belong with the REPL, but are a bit interwoven with other code in main.py.

Another desirable change might be migrating the handlers in key_binding_utils.py to repl.py or some new file repl_handlers.py.

We might also consider now removing sections of the relatively brittle tests in test_main_regression.py which relate to the REPL, and a note is left to that effect.

run_cli() is left in place for now, but the intention is to fully replace it with main_repl().

Checklist

• I added this contribution to the changelog.md file.
• I added my name to the AUTHORS file (or it's already there).
• To lint and format the code, I ran

  uv run ruff check && uv run ruff format && uv run mypy --install-types .