api: Enable usage of Arrays and Dictionaries as [k]vectors

- Rename a/n/m to items/size/capactity in kvec.h
- Add capactity field to Arrays/Dictionaries

src/nvim/api/private/defs.h

@@ -49,14 +49,14 @@ typedef struct {
 
 typedef struct {
   Object *items;
-  size_t size;
+  size_t size, capacity;
 } Array;
 
 typedef struct key_value_pair KeyValuePair;
 
 typedef struct {
   KeyValuePair *items;
-  size_t size;
+  size_t size, capacity;
 } Dictionary;
 
 typedef enum {