Enter HO NO, HO MO with ZWJ like other conjuncts

* src/lx-tn-im-table.c (lx_tn_im_conversion):
  - Also check for ZWJ (U+200D) and convert HO + ZWJ + {NO, MO} to
    {HO-NO, HO-MO}.
Simplify single-branch conditions

* src/lx-tn-im-table.c (lx_tn_im_conversion):
  - Simplify single-branch switch-block and if-block in the case of
    LO LING.
Change ZWJ key for easier access

* src/lx-tn-kbd.c (tn_keycode_map):
  - Revert change for qwerty's '&' key.
  - Change qwerty's '`' key to ZWJ.