[Lua] Add LuaJIT support (#6584)

* [Lua] Add LuaJIT support

Here is the output of LuaTest.sh:

Run with LuaJIT:
built 100 512-byte flatbuffers in 0.16sec: 0.63/msec, 0.31MB/sec
built 1000 512-byte flatbuffers in 0.08sec: 12.06/msec, 5.89MB/sec
built 10000 512-byte flatbuffers in 0.80sec: 12.44/msec, 6.07MB/sec
built 10000 512-byte flatbuffers in 0.33sec: 30.58/msec, 14.93MB/sec
traversed 100 592-byte flatbuffers in 0.04sec: 2.51/msec, 1.42MB/sec
traversed 1000 592-byte flatbuffers in 0.03sec: 31.52/msec, 17.79MB/sec
traversed 10000 592-byte flatbuffers in 0.21sec: 48.77/msec, 27.53MB/sec
Run with Lua 5.3:
built 100 512-byte flatbuffers in 0.02sec: 5.44/msec, 2.66MB/sec
built 1000 512-byte flatbuffers in 0.17sec: 5.74/msec, 2.80MB/sec
built 10000 512-byte flatbuffers in 1.75sec: 5.72/msec, 2.79MB/sec
built 10000 512-byte flatbuffers in 1.38sec: 7.26/msec, 3.55MB/sec
traversed 100 592-byte flatbuffers in 0.00sec: 27.64/msec, 15.60MB/sec
traversed 1000 592-byte flatbuffers in 0.03sec: 30.46/msec, 17.20MB/sec
traversed 10000 592-byte flatbuffers in 0.34sec: 29.62/msec, 16.72MB/sec

* [Lua] Better usage description

* update according to the review

lua/flatbuffers/compat.lua

@@ -1,17 +1,12 @@
-local m = {}
-
-local getAlignSize
+local m
 if _VERSION == "Lua 5.3" then
-    getAlignSize = function(k, size)
-            return ((~k) + 1) & (size - 1)
-        end    
+    m = require("flatbuffers.compat_5_3")
 else
-    getAlignSize = function(self, size, additionalBytes)        
-        local alignsize = bit32.bnot(#self.bytes-self:Head() + additionalBytes) + 1
-        return bit32.band(alignsize,(size - 1))
+    local ok = pcall(require, "jit")
+    if not ok then
+        error("Only Lua 5.3 or LuaJIT is supported")
+    else
+        m = require("flatbuffers.compat_luajit")
     end
 end
-    
-m.GetAlignSize = getAlignSize
-
 return m