Horizontal square brackets

Horizontal square brackets

I was looking for something completely different in the Unicode
Standard and found a note that U+23B4..U+23B6 can be used in terminal
applications with text printed vertically.  So obviously I had to add
them.

Apparently the same doesn't apply to the other horizontal brackets at
U+23DC..U+32E1, which are purely stretchy mathematical brackets and
not interesting to us.

The odd positioning in the character cell is based on the Unicode code
chart.