api: nvim_get_mode()

Asynchronous API functions are served immediately, which means pending
input could change the state of Nvim shortly after an async API function
result is returned.

nvim_get_mode() is different:
  - If RPCs are known to be blocked, it responds immediately (without
    flushing the input/event queue)
  - else it is handled just-in-time before waiting for input, after
    pending input was processed. This makes the result more reliable
    (but not perfect).

Internally this is handled as a special case, but _semantically_ nothing
has changed: API users never know when input flushes, so this internal
special-case doesn't violate that. As far as API users are concerned,
nvim_get_mode() is just another asynchronous API function.

In all cases nvim_get_mode() never blocks for more than the time it
takes to flush the input/event queue (~µs).

Note: This doesn't address #6166; nvim_get_mode() will provoke #6166 if
e.g. `d` is operator-pending.

Closes #6159

src/nvim/msgpack_rpc/helpers.c

@@ -76,7 +76,7 @@ typedef struct {
   size_t idx;
 } MPToAPIObjectStackItem;
 
-/// Convert type used by msgpack parser to Neovim own API type
+/// Convert type used by msgpack parser to Nvim API type.
 ///
 /// @param[in]  obj  Msgpack value to convert.
 /// @param[out]  arg  Location where result of conversion will be saved.