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_quickfix.vim

@@ -811,7 +811,7 @@ func Test_locationlist()
       " NOTE: problem 1:
       " intentionally not setting 'lnum' so that the quickfix entries are not
       " valid
-      call setloclist(0, qflist, ' ')
+      eval qflist->setloclist(0, ' ')
     endfor
 
     " Test A
@@ -1699,7 +1699,7 @@ endfunc
 
 func Test_setqflist_invalid_nr()
   " The following command used to crash Vim
-  call setqflist([], ' ', {'nr' : $XXX_DOES_NOT_EXIST})
+  eval []->setqflist(' ', {'nr' : $XXX_DOES_NOT_EXIST})
 endfunc
 
 func Test_setqflist_user_sets_buftype()