Extract pos.h from structs.h

This will make it much simpler to move the other types in structs.h
which depend on `pos_T` and `lpos_T`.

src/pos.h

@@ -0,0 +1,23 @@
+#ifndef NEOVIM_POS_H
+#define NEOVIM_POS_H
+
+/*
+ * position in file or buffer
+ */
+typedef struct {
+  linenr_T lnum;        /* line number */
+  colnr_T col;          /* column number */
+  colnr_T coladd;
+} pos_T;
+
+# define INIT_POS_T(l, c, ca) {l, c, ca}
+
+/*
+ * Same, but without coladd.
+ */
+typedef struct {
+  linenr_T lnum;        /* line number */
+  colnr_T col;          /* column number */
+} lpos_T;
+
+#endif /* NEOVIM_POS_H */