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/api/keysets.lua

@@ -155,5 +155,44 @@ return {
   create_augroup = {
     "clear";
   };
+  cmd = {
+    "cmd";
+    "range";
+    "count";
+    "reg";
+    "bang";
+    "args";
+    "magic";
+    "mods";
+    "nargs";
+    "addr";
+    "nextcmd";
+  };
+  cmd_magic = {
+    "file";
+    "bar";
+  };
+  cmd_mods = {
+    "silent";
+    "emsg_silent";
+    "sandbox";
+    "noautocmd";
+    "browse";
+    "confirm";
+    "hide";
+    "keepalt";
+    "keepjumps";
+    "keepmarks";
+    "keeppatterns";
+    "lockmarks";
+    "noswapfile";
+    "tab";
+    "verbose";
+    "vertical";
+    "split";
+  };
+  cmd_opts = {
+    "output";
+  };
 }