feat(api): implement nvim_{add,del}_user_command

Add support for adding and removing custom user commands with the Nvim
API.

src/nvim/ex_docmd.h

@@ -32,6 +32,26 @@ typedef struct {
   tasave_T tabuf;
 } save_state_T;
 
+typedef struct ucmd {
+  char_u *uc_name;              // The command name
+  uint32_t uc_argt;             // The argument type
+  char_u *uc_rep;               // The command's replacement string
+  long uc_def;                  // The default value for a range/count
+  int uc_compl;                 // completion type
+  cmd_addr_T uc_addr_type;      // The command's address type
+  sctx_T uc_script_ctx;         // SCTX where the command was defined
+  char_u *uc_compl_arg;         // completion argument if any
+  LuaRef uc_compl_luaref;       // Reference to Lua completion function
+  LuaRef uc_luaref;             // Reference to Lua function
+} ucmd_T;
+
+#define UC_BUFFER       1       // -buffer: local to current buffer
+
+extern garray_T ucmds;
+
+#define USER_CMD(i) (&((ucmd_T *)(ucmds.ga_data))[i])
+#define USER_CMD_GA(gap, i) (&((ucmd_T *)((gap)->ga_data))[i])
+
 #ifdef INCLUDE_GENERATED_DECLARATIONS
 # include "ex_docmd.h.generated.h"
 #endif