patch 8.1.0542: shiftwidth() does not take 'vartabstop' into account

Problem:    shiftwidth() does not take 'vartabstop' into account.
Solution:   Use the cursor position or a position explicitly passed.
            Also make >> and << work better with 'vartabstop'. (Christian
            Brabandt)
diff --git a/src/proto/edit.pro b/src/proto/edit.pro
index 9ba7164..768af3a 100644
--- a/src/proto/edit.pro
+++ b/src/proto/edit.pro
@@ -46,4 +46,5 @@
 void ins_scroll(void);
 void ins_horscroll(void);
 int ins_copychar(linenr_T lnum);
+colnr_T get_nolist_virtcol(void);
 /* vim: set ft=c : */