fix(timer): redefine local function as method

The previous commit added a private property, but they
cannot be set outside of class methods. Added an error
when starting a running timer, too.

imports/timer/shared.lua

@@ -35,12 +35,8 @@ function timer:constructor(time, onEnd, async)
     self:start(async)
 end
 
-function timer:start(async)
+---@protected
-    if self.private.startTime > 0 then return end
+function timer:run()
-
-    self.private.startTime = GetGameTimer()
-
-    local function tick()
     while self:isPaused() or self:getTimeLeft('ms') > 0 do
         Wait(0)
     end
@@ -52,10 +48,15 @@ function timer:start(async)
     self.private.triggerOnEnd = true
 end
 
-    if not async then return tick() end
+function timer:start(async)
+    if self.private.startTime > 0 then error('Cannot start a timer that is already running') end
+
+    self.private.startTime = GetGameTimer()
+
+    if not async then return self:run() end
 
     Citizen.CreateThreadNow(function()
-        tick()
+        self:run()
     end)
 end