Add vim_feedkeys API function

- New API function to push data to the typeahead buffer - this should
  equivalent to the vimscript feedkeys() function
- In Vim there was a --remote-send command to insert input into a
  Vim server instance. Besides accepting key sequences it also
  translated special keys such as <CR> or <Leader>, backslash notation
  is ignored. This commit backports the original Vim handler for
  --remote-send as a bool option for vim_feedkeys()
- vim-patch:0

src/nvim/api/vim.c

@@ -19,6 +19,8 @@
 #include "nvim/message.h"
 #include "nvim/eval.h"
 #include "nvim/misc2.h"
+#include "nvim/term.h"
+#include "nvim/getchar.h"
 
 #define LINE_BUFFER_SIZE 4096
 
@@ -47,6 +49,60 @@ void vim_command(String str, Error *err)
   try_end(err);
 }
 
+/// Pass input keys to Neovim
+///
+/// @param keys to be typed
+/// @param replace_tcodes If true replace special keys such as <CR> or <Leader>
+///           for compatibility with Vim --remote-send expressions
+/// @param remap If True remap keys
+/// @param typed Handle keys as if typed; otherwise they are handled as
+///           if coming from a mapping.  This matters for undo,
+///           opening folds, etc.
+void vim_feedkeys(String keys, Boolean replace_tcodes, Boolean remap,
+                Boolean typed, Error *err)
+{
+  char *ptr = NULL;
+  char *cpo_save = (char *)p_cpo;
+
+  if (replace_tcodes) {
+    // Set 'cpoptions' the way we want it.
+    //    B set - backslashes are *not* treated specially
+    //    k set - keycodes are *not* reverse-engineered
+    //    < unset - <Key> sequences *are* interpreted
+    //  The last but one parameter of replace_termcodes() is TRUE so that the
+    //  <lt> sequence is recognised - needed for a real backslash.
+    p_cpo = (char_u *)"Bk";
+    replace_termcodes((char_u *)keys.data, (char_u **)&ptr, false, true, true);
+    p_cpo = (char_u *)cpo_save;
+  } else {
+    ptr = keys.data;
+  }
+
+  if (ptr == NULL) {
+    set_api_error("Failed to eval expression", err);
+  } else {
+    // Add the string to the input stream.
+    // Can't use add_to_input_buf() here, we now have K_SPECIAL bytes.
+    //
+    // First clear typed characters from the typeahead buffer, there could
+    // be half a mapping there.  Then append to the existing string, so
+    // that multiple commands from a client are concatenated.
+    if (typebuf.tb_maplen < typebuf.tb_len) {
+        del_typebuf(typebuf.tb_len - typebuf.tb_maplen, typebuf.tb_maplen);
+    }
+    (void)ins_typebuf((char_u *)ptr, (remap ? REMAP_YES : REMAP_NONE),
+                    typebuf.tb_len, !typed, false);
+
+    // Let input_available() know we inserted text in the typeahead
+    // buffer. */
+    typebuf_was_filled = true;
+
+    if (replace_tcodes) {
+      free(ptr);
+    }
+  }
+}
+
 /// Evaluates the expression str using the vim internal expression
 /// evaluator (see |expression|).
 /// Dictionaries and lists are recursively expanded.