Exempt _ENV from 214 warning, tweak error (#64)

* test: Add test to exempt builtin underscore prefixes causing warning
* fix: Exempt special builtin _ENV from 214 warning
* chore: Tweak warning message for 214 to be more explicit

spec/unused_locals_spec.lua

@@ -88,6 +88,10 @@ end
 ]])
    end)
 
+   it("exempt _ENV from warning on usage with underscore prefix", function()
+      assert_warnings({}, [[ return function(_ENV) return type(_ENV) end ]])
+   end)
+
    it("detects unused implicit self", function()
       assert_warnings({
          {code = "212", name = "self", self = true, line = 2, column = 11, end_column = 11}

src/luacheck/stages/detect_unused_locals.lua

@@ -42,7 +42,7 @@ stage.warnings = {
       fields = {"name", "func", "secondary", "useless", "recursive", "mutually_recursive"}},
    ["212"] = {message_format = unused_arg_message_format, fields = {"name", "self"}},
    ["213"] = {message_format = "unused loop variable {name!}", fields = {"name"}},
-   ["214"] = {message_format = "used variable {name!}", fields = {"name"}},
+   ["214"] = {message_format = "used variable {name!} with unused hint", fields = {"name"}},
    ["221"] = {message_format = "variable {name!} is never set", fields = {"name", "secondary"}},
    ["231"] = {message_format = "variable {name!} is never accessed", fields = {"name", "secondary"}},
    ["232"] = {message_format = "argument {name!} is never accessed", fields = {"name"}},
@@ -152,7 +152,7 @@ local function detect_unused_local(chstate, var)
    elseif #var.values == 1 then
       local value = var.values[1]
 
-      if var.hint_unused then
+      if var.hint_unused and var.name ~= "_ENV" then
          if value.used then
             chstate:warn_var("214", var)
          end