keyboard: add header navigation functionality
toc: use the location-list instead of the quickfix-list
keyboard: make CurrentHeaderParent more robust
keyboard: implement sibling header navigation
keyboard: implement children header navigation
keyboard: implement a method to apply a header level to a line
keyboard: implement a method to apply a header level to a line

also, implemented a method to remove them.

the behavior of this feature is configured through the
`g:pantondoc_keyboard_header_style` variable.