Most code in keymap.h is for keycode definitions, while most code in
keymap.c is for the parsing and conversion of keycodes.

The name "keymap" may also make people think these two files are for
mappings, while in fact keycodes are used even when no mappings are
involved, so "keycodes" should be a better file name than "keymap".

Problem:    GUI tests fail because the test doesn't use a modifier.
Solution:   Add "\{xxx}" to be able to encode a modifier.
https://github.com/vim/vim/commit/ebe9d34aa07037cff2188a8dd424ee1f59cbb0bf

Change macros to enums to use them in unit tests.

Problem:    Cannot map <C-H> when modifyOtherKeys is enabled.
Solution:   Add the <C-H> mapping twice, both with modifier and as 0x08.  Use
            only the first one when modifyOtherKeys has been detected.
https://github.com/vim/vim/commit/459fd785e4a8d044147a3f83a5fca8748528aa84

Add REPTERM_NO_SPECIAL instead of REPTERM_SPECIAL because the meaning of
"special" is different between Vim and Nvim.
Omit seenModifyOtherKeys as Nvim supports attaching multiple UIs.
Omit tests as they send terminal codes.
Keep the behavior of API functions.

cb02137dfac7 had two mistakes, of the same nature: trans_special() must
be invoked with in_string=false unless the parsing context is a VimL
string. replace_termcodes() and input_enqueue() are low-level
mechanisms where VimL strings do not exist.

keymap.c: adjust double-quote case to satisfy keymap_spec.lua

closes #7410