-1 is index past the end, and -2 is the index of the last element.
This eliminates the need for include_start/include_end.

Allow the handling of out-of-bounds to be configurable.

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

On Windows the default file format is DOS i.e. newlines are \r\n
instead of \n.

When converting a msgpack object to a String object, strings (and byte
arrays) with length 0 are handled as errors. This is fixed by
always using the msgpack data pointer as a valid pointer. For a NULL
pointer there is nothing to copy.

Test by @snoe

Fixes #3844

Refactor input.c, normal.c and edit.c to use the K_EVENT special key to trigger
the CURSORHOLD event. In normal and edit mode, K_EVENT is treated as
K_CURSORHOLD, which enables better handling of arbitrary actions in those
states(eg: In normal mode the previous operator counts will be restored).

Also fix a test in vim_spec.lua. The test had a wrong assumption: cmdheight is
only used to determine when the press enter screen will be shown, not to limit
how many lines or control pagination.