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vim-patch:8.2.3643: header for source file is outdated

Problem:    Header for source file is outdated.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/main.c

@@ -23,7 +23,6 @@
 #include "nvim/ex_getln.h"
 #include "nvim/fileio.h"
 #include "nvim/fold.h"
-#include "nvim/getchar.h"
 #include "nvim/hashtab.h"
 #include "nvim/highlight.h"
 #include "nvim/highlight_group.h"
@@ -31,6 +30,7 @@
 #include "nvim/if_cscope.h"
 #include "nvim/lua/executor.h"
 #include "nvim/main.h"
+#include "nvim/mapping.h"
 #include "nvim/ui_client.h"
 #include "nvim/vim.h"
 #ifdef HAVE_LOCALE_H