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/pos.h

@@ -1,10 +1,12 @@
 #ifndef NVIM_POS_H
 #define NVIM_POS_H
 
+#include <inttypes.h>
+
 /// Line number type
-typedef long linenr_T;
+typedef int32_t linenr_T;
 /// Format used to print values which have linenr_T type
-#define PRIdLINENR "ld"
+#define PRIdLINENR PRId32
 
 /// Column number type
 typedef int colnr_T;