feat(api): add nvim_cmd

Adds the API function `nvim_cmd` which allows executing an Ex-command through a Dictionary which can have the same values as the return value of `nvim_parse_cmd()`. This makes it much easier to do things like passing arguments with a space to commands that otherwise may not allow it, or to make commands interpret certain characters literally when they otherwise would not.

src/nvim/ex_cmds_defs.h

@@ -175,6 +175,9 @@ enum {
 /// Arguments used for Ex commands.
 struct exarg {
   char *arg;                    ///< argument of the command
+  char **args;                  ///< starting position of command arguments
+  size_t *arglens;              ///< length of command arguments
+  size_t argc;                  ///< number of command arguments
   char *nextcmd;                ///< next command (NULL if none)
   char *cmd;                    ///< the name of the command (except for :make)
   char **cmdlinep;              ///< pointer to pointer of allocated cmdline