Add tests for:
- Cursor position restored after :map expr
- Cursor position restored after :imap expr
- Error in :cmap expr handled correctly
Cherry-picked from #12837
cb02137dfa 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
Note: there are three changes to ascii_isident. Reverting first two (in
find_special_key and first in get_special_key_code) normally fails the new test
with empty &isident, but reverting the third does not. Hence adding `>` to
&isident.
Ref vim/vim#2389.
