vim-patch:8.2.4932: not easy to filter the output of maplist()

Problem:    Not easy to filter the output of maplist().
Solution:   Add mode_bits to the dictionary. (Ernie Rael, closes vim/vim#10356)

d8f5f76621

Co-authored-by: Ernie Rael <errael@raelity.com>

runtime/lua/vim/_meta/vimfn.lua

@@ -5047,6 +5047,11 @@ function vim.fn.map(expr1, expr2) end
 ---   "nowait"   Do not wait for other, longer mappings.
 ---        (|:map-<nowait>|).
 ---   "abbr"     True if this is an |abbreviation|.
+---   "mode_bits" Nvim's internal binary representation of "mode".
+---        |mapset()| ignores this; only "mode" is used.
+---        See |maplist()| for usage examples. The values
+---        are from src/nvim/vim.h and may change in the
+---        future.
 ---
 --- The dictionary can be used to restore a mapping with
 --- |mapset()|.
@@ -5111,6 +5116,26 @@ function vim.fn.mapcheck(name, mode, abbr) end
 ---   echo maplist()->filter({_, m ->
 ---     \ match(get(m, 'rhs', ''), 'MultiMatch') >= 0
 ---     \ })
+--- <It can be tricky to find mappings for particular |:map-modes|.
+--- |mapping-dict|'s "mode_bits" can simplify this. For example,
+--- the mode_bits for Normal, Insert or Command-line modes are
+--- 0x19. To find all the mappings available in those modes you
+--- can do: >vim
+---   let saved_maps = []
+---   for m in maplist()
+---       if and(m.mode_bits, 0x19) != 0
+---     eval saved_maps->add(m)
+---       endif
+---   endfor
+---   echo saved_maps->mapnew({_, m -> m.lhs})
+--- <The values of the mode_bits are defined in Nvim's
+--- src/nvim/vim.h file and they can be discovered at runtime
+--- using |:map-commands| and "maplist()". Example: >vim
+---   omap xyzzy <Nop>
+---   let op_bit = maplist()->filter(
+---       \ {_, m -> m.lhs == 'xyzzy'})[0].mode_bits
+---   ounmap xyzzy
+---   echo printf("Operator-pending mode bit: 0x%x", op_bit)
 ---
 --- @return any
 function vim.fn.maplist() end