Configurable balanced-bracket highlight colors

[rolandwalker]

Description

Make the balanced-bracket highlight feature use configurable colors, setting them by default to the prompt_toolkit defaults.

Non-default example:

Motivation: the default background highlight can be confused with the block cursor.

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 .