`<Esc>` in insert mode returns to normal mode`i<BS>` on line zero beeps
`i<BS>` at 1,0 joins lines`i<BS>` at 1,0 joins lines
Move <BS>-at-BOL logic into b/backspace
`i<BS>` at 1,0 leaves the cursor at join`i<BS>` at 1,0 leaves the cursor at join
Extract `backspace-at-beginning-of-line`