feat: Support promotion instead of deletion the last empty bullet

[devsunb]

Support promotion instead of deletion the last empty bullet when hitting <cr> (insert mode) or o (normal mode).
Fixes #155

[dkarter]

Thank you for this PR! I like the approach, but have two notes:

1. Functional: to fully address the issue this is closing the behavior of = 2 should be that if it's the last bullet after we de-indented on the next return it should delete the bullet - this will be in line with the apps mentioned as an example (Obsidian/Google Docs etc).

2. Super minor/docs: isn't it "demote" instead of "promote"? I could be wrong, but either way that terminology is a bit confusing. Maybe we should use "indent" and "deindent"? What do you think?

[devsunb]

1. It seems to work as you said. (same as :BulletPromote)

# cursor on |
- a
  - b
  - |

# after <cr>
- a
  - b
- |

# after <cr> again
- a
  - b
|

2. I agree, but since this plugin consistently uses the terms Demote/Promote, I think it would be better to maintain consistency.

[dkarter]

Thank you! 🫶

[linkarzu]

Awesome, really appreciate this!