core: don't rely on sane global environment

some addons like to provide a broken _G variable to
the global environment breaking every other addon that
relies on it if existing. With this commit, make ShaguTweaks
no longer re-use the global _G, even if existing.

helpers.lua

@@ -1,6 +1,29 @@
-local _G = _G or getfenv(0)
 local gfind = string.gmatch or string.gfind
 
+ShaguTweaks.GetExpansion = function()
+  local _, _, _, client = GetBuildInfo()
+  client = client or 11200
+
+  -- detect client expansion
+  if client >= 20000 and client <= 20400 then
+    return "tbc"
+  elseif client >= 30000 and client <= 30300 then
+    return "wotlk"
+  else
+    return "vanilla"
+  end
+end
+
+ShaguTweaks.GetGlobalEnv = function()
+  if ShaguTweaks.GetExpansion() == 'vanilla' then
+    return getfenv(0)
+  else
+    return _G or getfenv(0)
+  end
+end
+
+local _G = ShaguTweaks.GetGlobalEnv()
+
 local gradientcolors = {}
 ShaguTweaks.GetColorGradient = function(perc)
   perc = perc > 1 and 1 or perc