feat(lua): allow :=expr as a shorter version of :lua =expr

existing behavior of
  :=
and
  :[range]=

are unchanged. `|` is still allowed with this usage.

However,
   :=p
and similar are changed in a way which could be construed as a breaking
change. Allowing |ex-flags| for := in the first place was a mistake as
any form of := DOES NOT MOVE THE CURSOR. So it would print one line number
and then print a completely different line contents after that.

test/functional/lua/commands_spec.lua

@@ -146,6 +146,7 @@ describe(':lua command', function()
   it('prints result of =expr', function()
     exec_lua("x = 5")
     eq("5", exec_capture(':lua =x'))
+    eq("5", exec_capture('=x'))
     exec_lua("function x() return 'hello' end")
     eq('hello', exec_capture(':lua = x()'))
     exec_lua("x = {a = 1, b = 2}")
@@ -165,7 +166,7 @@ describe(':lua command', function()
       false
       nil
       Error message]],
-    exec_capture(':lua  =x(false)'))
+    exec_capture('=x(false)'))
   end)
 end)