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_functions.vim

@@ -1219,7 +1219,7 @@ func Test_shellescape()
   let save_shell = &shell
   set shell=bash
   call assert_equal("'text'", shellescape('text'))
-  call assert_equal("'te\"xt'", shellescape('te"xt'))
+  call assert_equal("'te\"xt'", 'te"xt'->shellescape())
   call assert_equal("'te'\\''xt'", shellescape("te'xt"))
 
   call assert_equal("'te%xt'", shellescape("te%xt"))