refactor: change type of linenr_T from long to int32_t

The size of long varies depending on architecture, in contrast to the
MAXLNUM constant which sets the maximum allowable number of lines to
2^32-1. This discrepancy may lead to hard to detect bugs, for example
https://github.com/neovim/neovim/issues/18454. Setting linenr_T to a
fix maximum size of 2^32-1 will prevent this type of errors in the
future.

Also change the variables `amount` and `amount_after` to be linenr_T
since they're referring to "the line number difference" between two
texts.

src/nvim/api/win_config.c

@@ -325,7 +325,7 @@ static bool parse_float_bufpos(Array bufpos, lpos_T *out)
       || bufpos.items[1].type != kObjectTypeInteger) {
     return false;
   }
-  out->lnum = bufpos.items[0].data.integer;
+  out->lnum = (linenr_T)bufpos.items[0].data.integer;
   out->col = (colnr_T)bufpos.items[1].data.integer;
   return true;
 }