Add a Markdown navigation pane

[kisaraki]

Summary

• add a View > Navigation Pane command that is disabled for non-.md files
• parse ATX headings from # through ###### and render a six-level tree
• summarize intervening non-heading content as [......]
• support mouse/keyboard activation, cursor jumps, active-row highlighting, and +/- folding
• center the pane title and keep the ESC hint at the bottom

Validation

• unit tests cover six heading levels, fenced-code exclusion, asterisk rejection, content grouping, and cursor jumps
• cargo test --all-features --all-targets on Windows and WSL/Ubuntu
• cargo clippy --workspace --all-features --all-targets -- --no-deps --deny warnings on Windows and WSL/Ubuntu

Contribution ownership

This contribution is entirely my own work. It is not created on behalf of, or connected with, any organization or company. I agree to contribute it under the repository's MIT License and to the repository's applicable contribution terms.