refactor(api/nvim_cmd): use kvec_t for constructing cmdline string

Co-authored-by: Björn Linse <bjorn.linse@gmail.com>

src/nvim/strings.h

@@ -7,6 +7,7 @@
 
 #include "nvim/eval/typval.h"
 #include "nvim/types.h"
+#include "nvim/lib/kvec.h"
 
 /// Append string to string and return pointer to the next byte
 ///
@@ -25,6 +26,8 @@ static inline char *strappend(char *const dst, const char *const src)
   return (char *)memmove(dst, src, src_len) + src_len;
 }
 
+typedef kvec_t(char) StringBuilder;
+
 #ifdef INCLUDE_GENERATED_DECLARATIONS
 # include "strings.h.generated.h"
 #endif