vim-patch:9.1.0899: default for 'backspace' can be set in C code (#31416)

Problem:  default for 'backspace' can be set in C code
Solution: promote the default for 'backspace' from defaults.vim to the C
          code (Luca Saccarola)

closes: vim/vim#16143

959ef61430

N/A patches:
vim-patch:9.1.0895: default history value is too small
vim-patch:075aeea: runtime(doc): document changed default value for 'history'

Co-authored-by: Luca Saccarola <github.e41mv@aleeas.com>

test/old/testdir/test_digraph.vim

@@ -250,9 +250,12 @@ func Test_digraphs_option()
   call Put_Dig_BS("P","=")
   call assert_equal(['Р']+repeat(["₽"],2)+['П'], getline(line('.')-3,line('.')))
   " Not a digraph: this is different from <c-k>!
+  let _bs = &bs
+  set bs=
   call Put_Dig_BS("a","\<bs>")
   call Put_Dig_BS("\<bs>","a")
   call assert_equal(['','a'], getline(line('.')-1,line('.')))
+  let &bs = _bs
   " Grave
   call Put_Dig_BS("a","!")
   call Put_Dig_BS("!","e")