vim-patch:8.2.3184: cannot add a digraph with a leading space

Problem:    Cannot add a digraph with a leading space.  It is not easy to list
            existing digraphs.
Solution:   Add setdigraph(), setdigraphlist(), getdigraph() and
            getdigraphlist(). (closes vim/vim#8580)
6106504e9e

Use GA_APPEND_VIA_PTR in registerdigraph().
Use tv_list_append_*() in getdigraphlist_appendpair().
Put the error messages in digraph.c.
E196 is N/A.
Remove mentions about 'encoding' being non-Unicode.
Nvim doesn't support setting encoding=japan, so skip a test.

src/nvim/eval.lua

@@ -156,6 +156,8 @@ return {
     getcurpos={args={0, 1}, base=1},
     getcursorcharpos={args={0, 1}, base=1},
     getcwd={args={0, 2}, base=1},
+    getdigraph={args=1, base=1},
+    getdigraphlist={args={0, 1}, base=1},
     getenv={args=1, base=1},
     getfontname={args={0, 1}},
     getfperm={args=1, base=1},
@@ -322,6 +324,8 @@ return {
     setcharsearch={args=1, base=1},
     setcmdpos={args=1, base=1},
     setcursorcharpos={args={1, 3}, base=1},
+    setdigraph={args=2, base=1},
+    setdigraphlist={args=1, base=1},
     setenv={args=2, base=2},
     setfperm={args=2, base=1},
     setline={args=2, base=2},