patch 8.1.1996: more functions can be used as methods

Problem:    More functions can be used as methods.
Solution:   Make various functions usable as a method.
diff --git a/src/testdir/test_vartabs.vim b/src/testdir/test_vartabs.vim
index b0f3e15..47844fb 100644
--- a/src/testdir/test_vartabs.vim
+++ b/src/testdir/test_vartabs.vim
@@ -329,7 +329,7 @@
   let lines = ScreenLines([1, 2], winwidth(0))
   call s:compare_lines(expect2, lines)
   call assert_equal(20, shiftwidth(virtcol('.')-2))
-  call assert_equal(30, shiftwidth(virtcol('.')))
+  call assert_equal(30, virtcol('.')->shiftwidth())
   norm! $>>
   let expect3 = ['                                        ', '                    x                   ', '~                                       ']
   let lines = ScreenLines([1, 3], winwidth(0))