vim-patch:8.1.1210: support for user commands is spread out (#19653)

Problem:    Support for user commands is spread out. No good reason to make
            user commands optional.
Solution:   Move user command support to usercmd.c.  Always enable the
            user_commands feature.
ac9fb18020

src/nvim/usercmd.h

@@ -0,0 +1,30 @@
+#ifndef NVIM_USERCMD_H
+#define NVIM_USERCMD_H
+
+#include "nvim/ex_cmds_defs.h"
+
+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_preview_luaref;     // Reference to Lua preview 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 "usercmd.h.generated.h"
+#endif
+#endif  // NVIM_USERCMD_H