feat(eval/method): partially port v8.1.1921

Adds method call support for all functions in the patch, but it cannot
be fully ported due to missing tests for:

- filereadable(): requires v8.1.1378 for Test_delete_rf(), but there
                  appears to have been some trouble porting it. (#12784)

- confirm(): requires v8.1.0832 for Test_confirm() and v8.1.0815 for
             feedkeys()'s "L" flag.

             (I did attempt to port the test using nvim_input() instead,
             but seems that input handling for confirm() doesn't work in
             --headless mode?)

             Note that confirm() was actually added as a method in
             v8.1.1915.

Uncomment use of method call syntax in Test_Executable() previously
included instead from v8.2.2259.

src/nvim/testdir/test_vimscript.vim

@@ -635,7 +635,7 @@ function! MSG(enr, emsg)
 	if v:errmsg == ""
 	    Xout "Message missing."
 	else
-	    let v:errmsg = escape(v:errmsg, '"')
+	    let v:errmsg = v:errmsg->escape('"')
 	    Xout "Unexpected message:" v:errmsg
 	endif
     endif