vim-patch:8.1.1823: command line history code is spread out (#19688)

Problem:    Command line history code is spread out.
Solution:   Put the code in a new file. (Yegappan Lakshmanan, closes vim/vim#4779)
            Also graduate the +cmdline_hist feature.
d7663c22c6

src/nvim/cmdhist.h

@@ -0,0 +1,33 @@
+#ifndef NVIM_CMDHIST_H
+#define NVIM_CMDHIST_H
+
+#include "nvim/eval/typval.h"
+#include "nvim/ex_cmds_defs.h"
+#include "nvim/os/time.h"
+
+/// Present history tables
+typedef enum {
+  HIST_DEFAULT = -2,  ///< Default (current) history.
+  HIST_INVALID = -1,  ///< Unknown history.
+  HIST_CMD = 0,       ///< Colon commands.
+  HIST_SEARCH,        ///< Search commands.
+  HIST_EXPR,          ///< Expressions (e.g. from entering = register).
+  HIST_INPUT,         ///< input() lines.
+  HIST_DEBUG,         ///< Debug commands.
+} HistoryType;
+
+/// Number of history tables
+#define HIST_COUNT      (HIST_DEBUG + 1)
+
+/// History entry definition
+typedef struct hist_entry {
+  int hisnum;           ///< Entry identifier number.
+  char_u *hisstr;       ///< Actual entry, separator char after the NUL.
+  Timestamp timestamp;  ///< Time when entry was added.
+  list_T *additional_elements;  ///< Additional entries from ShaDa file.
+} histentry_T;
+
+#ifdef INCLUDE_GENERATED_DECLARATIONS
+# include "cmdhist.h.generated.h"
+#endif
+#endif  // NVIM_CMDHIST_H