hterm: Add basic backspace support in hterm.html
hterm: Add basic backspace support in hterm.html

This backspace works when the cursor is advanced,
but does not handle unwrapping back to a previous line.

Change-Id: I7abf1fb2f5c8adb448d53053ec99402e7f9bd2b7
Reviewed-on: https://chromium-review.googlesource.com/c/apps/libapps/+/1871796
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Mike Frysinger <8f3f75c74bd5184edcfa6534cab3c13a00a2f794@chromium.org>

chromium/hterm,chromium/hterm,chromium/hterm,chromium/hterm