Add ‘eob’ option to fillchars

This option allows configuring what character is shown on the empty
lines at the end of a buffer, previously hardcoded to ‘~’

src/nvim/option.c

@@ -3386,6 +3386,7 @@ static char_u *set_chars_option(char_u **varp)
     { &fill_fold,    "fold" , 183  },  // ·
     { &fill_diff,    "diff" , '-'  },
     { &fill_msgsep,  "msgsep", ' ' },
+    { &fill_eob,     "eob",   '~' },
   };
   static struct charstab lcstab[] = {
     { &lcs_eol,      "eol",      NUL },