vim: move linenr_T and colnr_T to pos.h

Try to cut down vim.h's size. It's keeping us from testing more things.

src/nvim/pos.h

@@ -1,6 +1,12 @@
 #ifndef NVIM_POS_H
 #define NVIM_POS_H
 
+typedef long linenr_T;         // line number type
+typedef int colnr_T;           // column number type
+
+#define MAXLNUM (0x7fffffffL)  // maximum (invalid) line number
+#define MAXCOL  (0x7fffffffL)  // maximum column number, 31 bits
+
 /*
  * position in file or buffer
  */