Adds support for:
- api:vim_input("<D-a>")
- ":nnoremap <C-D-S-...>" and permutations thereof
UIs must capture the modifier and send it as "<D-...>" to vim_input().
Note: Before this commit, any arbitrary ":nnoremap <{foo}-{bar}>"
mapping could already be invoked with feedkeys("\<{foo}-{bar}>"). This
commit supports "D-" as a modifier that can be combined with "C-", "A-",
"S-" in any order.
For non-GUI (terminal) support, user must:
:set <D-a>={CSI sequence}
then send the {CSI sequence} from their terminal. But this does not work
yet (regression #2204).
Closes#2190
99d4c8c29c