feat(eval/method): partially port v8.1.1993

Problem:    More functions can be used as methods.
Solution:   Make various functions usable as a method.
196b466443

server2client requires +clientserver, which hasn't been ported yet.
The eval.txt docs and test_clientserver.vim tests for server2client already exist, so include those
changes.

test_bufline.vim: Test for setbufline requires v8.1.1189 (which was reverted in #10848).

test/functional/legacy/searchpos_spec.lua

@@ -17,7 +17,7 @@ describe('searchpos', function()
     call('cursor', 1, 1)
     eq({1, 1, 2}, eval([[searchpos('\%(\([a-z]\)\|\_.\)\{-}xyz', 'pcW')]]))
     call('cursor', 1, 2)
-    eq({2, 1, 1}, eval([[searchpos('\%(\([a-z]\)\|\_.\)\{-}xyz', 'pcW')]]))
+    eq({2, 1, 1}, eval([['\%(\([a-z]\)\|\_.\)\{-}xyz'->searchpos('pcW')]]))
 
     command('set cpo-=c')
     call('cursor', 1, 2)