Completion on abbreviated pathname elements

[rolandwalker]

Description

Now that pathname completions are working at all, we can add a popular completion method adapted from fish. The example path

./directory/subdirectory/example.sql

can be completed on leading abbreviations after each slash, including abbreviations of a single letter, such as

./d/s/e<tab>

This abbreviation method applies only to pathnames.

xref #2211.

Example:

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 .