refactor: move some mapping-related code to a separate file (#19061)

This marks the following Vim patches as ported:

vim-patch:8.1.1785: map functionality mixed with character input

Problem:    Map functionality mixed with character input.
Solution:   Move the map functionality to a separate file. (Yegappan
            Lakshmanan, closes vim/vim#4740)  Graduate the +localmap feature.
b66bab381c

vim-patch:8.2.3643: header for source file is outdated

Problem:    Header for source file is outdated.
Solution:   Make the header more accurate. (closes vim/vim#9186)
a3f83feb63

Also cherry-pick a change for <unique> mappings from patch 8.2.0807.
Rename map_clear_mode() to do_mapclear().

src/nvim/mapping.h

@@ -0,0 +1,54 @@
+#ifndef NVIM_MAPPING_H
+#define NVIM_MAPPING_H
+
+#include "nvim/buffer_defs.h"
+#include "nvim/eval/funcs.h"
+#include "nvim/ex_cmds_defs.h"
+#include "nvim/types.h"
+#include "nvim/vim.h"
+
+/// All possible |:map-arguments| usable in a |:map| command.
+///
+/// The <special> argument has no effect on mappings and is excluded from this
+/// struct declaration. |noremap| is included, since it behaves like a map
+/// argument when used in a mapping.
+///
+/// @see mapblock_T
+struct map_arguments {
+  bool buffer;
+  bool expr;
+  bool noremap;
+  bool nowait;
+  bool script;
+  bool silent;
+  bool unique;
+
+  /// The {lhs} of the mapping.
+  ///
+  /// vim limits this to MAXMAPLEN characters, allowing us to use a static
+  /// buffer. Setting lhs_len to a value larger than MAXMAPLEN can signal
+  /// that {lhs} was too long and truncated.
+  char_u lhs[MAXMAPLEN + 1];
+  size_t lhs_len;
+
+  /// Unsimplifed {lhs} of the mapping. If no simplification has been done then alt_lhs_len is 0.
+  char_u alt_lhs[MAXMAPLEN + 1];
+  size_t alt_lhs_len;
+
+  char_u *rhs;  /// The {rhs} of the mapping.
+  size_t rhs_len;
+  LuaRef rhs_lua;  /// lua function as {rhs}
+  bool rhs_is_noop;  /// True when the {rhs} should be <Nop>.
+
+  char_u *orig_rhs;  /// The original text of the {rhs}.
+  size_t orig_rhs_len;
+  char *desc;  /// map description
+};
+typedef struct map_arguments MapArguments;
+#define MAP_ARGUMENTS_INIT { false, false, false, false, false, false, false, \
+                             { 0 }, 0, { 0 }, 0, NULL, 0, LUA_NOREF, false, NULL, 0, NULL }
+
+#ifdef INCLUDE_GENERATED_DECLARATIONS
+# include "mapping.h.generated.h"
+#endif
+#endif  // NVIM_MAPPING_H