feat(eval/method): partially port v8.1.1996

Problem:    More functions can be used as methods.
Solution:   Make various functions usable as a method.
aad222c9c9

Partial port as this does not include eval.txt change for listener_add.

Cherry-pick eval.txt changes for:
- bufadd from v8.1.1626:
  5ca1ac373a
- setloclist and setqflist headers from:
  b0d45e7f53

Correct eval.txt typo for settabwinvar method call (matches latest Vim).

src/nvim/testdir/test_expr.vim

@@ -493,7 +493,7 @@ func Test_setmatches()
     let set[0]['conceal'] = 5
     let exp[0]['conceal'] = '5'
   endif
-  call setmatches(set)
+  eval set->setmatches()
   call assert_equal(exp, getmatches())
 endfunc