Fix: Add multiple checkboxes at once

[codebykat]

Fix

This restores the ability to select multiple lines and toggle checkboxes on all of them.

n.b. this runs toggle checkbox on all lines of selection, meaning if you select some lines that already have a checkbox, the checkbox will be removed. Is this the desired behavior?

Fixes #2409

Test

1. Select multiple lines, Insert Checklist should run the "toggle checkbox" on each of them (inserting or removing depending on if it had one already)
2. Without a selection, Insert Checklist should toggle checkbox on the current line (where the cursor is)

Release

Restored ability to select multiple lines and add multiple checkboxes at once with Insert Checklist

[belcherj]

Looks great. The only problem I see is if you have a mix of checkboxes and no checkboxes in a selection. I wonder if we should toggle all lines in a selection the opposite of the first line in the selection. Something to think about for another PR.