mode/prompt-buffer: Add delete-backwards-word vi keybinding

[MaxGyver83]

Description

Add missing delete-backwards-word vi keybinding to prompt-buffer.

Discussion

Not much to discuss:

db delete from cursor to previous start of word

→ vim help

Checklist:

Everything in this checklist is required for each PR. Please do not approve a PR that does not have all of these items.

• Git hygiene:

  • I have pulled from master before submitting this PR
  • There are no merge conflicts.

• My code follows the style guidelines for Common Lisp code. See:

  • Norvig & Pitman's Tutorial on Good Lisp Programming Style (PDF)
  • Google Common Lisp Style Guide

• I have performed a self-review of my own code.

• My code has been reviewed by at least one peer. (The peer review to approve a PR counts. The reviewer must download and test the code.)

• Documentation:

  • All my code has docstrings and :documentations written in the aforementioned style. (It's OK to skip the docstring for really trivial parts.)
  • I have updated the existing documentation to match my changes.
  • I have commented my code in hard-to-understand areas.
  • I have updated the changelog.lisp with my changes if it's anything user-facing (new features, important bug fix, compatibility breakage).
    • Changelog update should be a separate commit.
  • I have added a migration.lisp entry for all compatibility-breaking changes.
  • (If this changes something about the features showcased on Nyxt website) I have these changes described in the new/existing article at Nyxt website or will notify one of maintainters to do so.

• Compilation and tests:

  • My changes generate no new warnings.
  • I have added tests that prove my fix is effective or that my feature works. (If possible.)
  • I ran the tests locally ((asdf:test-system :nyxt) and (asdf:test-system :nyxt/gi-gtk)) and they pass.

[aadcg]

@MaxGyver83, thank you for your interest! Your feedback and thoughts are always appreciated.