ftplugin/sub_qc.vim: Add `[[`, `[]`, `]]`, `][` commands
Create mappings for these section motion commands and a function that
performs the motion.
The start of a section is defined as one line after the `###…`
separator. The end of a section is defined as 2 lines above the `###…`
separator.
Note that this code currently has a bug where if the current line is at
1. the start of a section and you try to go back to the start of the
previous section or
2. the end of a section and you try to go forward to the end of the next
section,
the cursor won't move. This needs to be fixed so that in both of these
cases the cursor moves to the appropriate position.