feat(lsp): deprecate non-method client functions

Deprecated:
- `client.request()` -> `client:request()`
- `client.request_sync()` -> `client:request_sync()`
- `client.notify()` -> `client:notify()`
- `client.cancel_request()` -> `client:cancel_request()`
- `client.stop()` -> `client:stop()`
- `client.is_stopped()` `client:is_stopped()`
- `client.supports_method()` -> `client:supports_method()`
- `client.on_attach()` -> `client:on_attach()`

Fixed docgen to link class fields to the full function doc.

test/functional/plugin/lsp/testutil.lua

@@ -182,16 +182,17 @@ function M.test_rpc_server(config)
     )
   end
   local client = setmetatable({}, {
-    __index = function(_, name)
+    __index = function(t, name)
       -- Workaround for not being able to yield() inside __index for Lua 5.1 :(
       -- Otherwise I would just return the value here.
-      return function(...)
+      return function(arg1, ...)
+        local ismethod = arg1 == t
         return exec_lua(function(...)
-          if type(_G.TEST_RPC_CLIENT[name]) == 'function' then
-            return _G.TEST_RPC_CLIENT[name](...)
-          else
-            return _G.TEST_RPC_CLIENT[name]
+          local client = _G.TEST_RPC_CLIENT
+          if type(client[name]) == 'function' then
+            return client[name](ismethod and client or arg1, ...)
           end
+          return client[name]
         end, ...)
       end
     end,