api: Implement vim_command_output function

This function can be used by API clients to execute a command and capture the
output.

src/nvim/eval.h

@@ -64,6 +64,7 @@ enum {
     VV_WINDOWID,
     VV_PROGPATH,
     VV_JOB_DATA,
+    VV_COMMAND_OUTPUT,
     VV_LEN, /* number of v: vars */
 };