add RecomputeCheckboxes command

[xs]

Description

Here we add a new RecomputeCheckboxes command that scans the current list under the cursor, then recomputes partials, preserving checkbox leaf state (and uses those leaves as a source of truth).

Rationale

Partial checkbox computation is currently only triggered when checkboxes are toggled (via toggle_checkboxes_nested() via the ToggleCheckbox command), but there are a lot of ways that underlying state can change without triggering computation, including:

• inserting lines with new checkbox bullets
• deleting lines with checkbox bullets
• manually editing checkbox content in insert mode

Here I've minimally added the RecomputeCheckboxes command to allow a full compute of checkboxes in the list.

Edge cases

Partial checkboxes without children

I've decided to treat these as unchecked (with the assumption that it's better for for users to manually mark these as checked themselves if necessary).

Non-checkbox content in lists

To make the minimal change in this PR, I'm leaving the current behavior: non-checkbox children are treated as unchecked checkboxes. For example, the following lists will be recomputed as follows:

- [x] parent
  - [x] checked child
  - [x] checked child
  
- [o] parent
  - [x] checked child
  - non-checkbox content
  - [x] checked child

I'd like to follow-up with another PR that allows s:sibling_checkbox_status() to handle non-checkbox content, but the change is a bit more involved (since it requires a bit of checking to avoid division-by-zero).

bullets.vim/plugin/bullets.vim

Lines 1468 to 1480 in 8f420b5

	for l:lnum in l:siblings
	let l:indent = indent(l:lnum)
	let l:bullet = s:closest_bullet_types(l:lnum, l:indent)
	let l:bullet = s:resolve_bullet_type(l:bullet)
	if !empty(l:bullet) && has_key(l:bullet, 'checkbox_marker')
	let l:checkbox_content = l:bullet.checkbox_marker
	if l:checkbox_content =~# '\v[xX' . l:checkbox_markers[-1] . ']'
	" Checked
	let l:checked += 1
	endif
	endif
	endfor

Possible future work

Having this function can enable something like a bullets_recompute_checkboxes_on_change config variable that functions similarly to bullets_renumber_on_change, but that can likewise be handled in a future PR.

Thank you for reviewing!

[harshad1]

Looks good. Merging.

Apologies for the delay getting to this.