[lua] Implement Sys.getEnv and Timer.stamp for lua-vanilla mode

Sys.getEnv() was throwing NotImplementedException in lua-vanilla mode
despite having a trivial vanilla Lua implementation via os.getenv().
Timer.stamp() now uses os.clock() directly on all Lua targets for
subsecond precision.

Sys.time() is intentionally left as notImplemented() in lua-vanilla
mode: vanilla Lua's os.time() has only second-level precision, which
would silently disagree with the fractional-second wall-clock time
returned by Sys.time() on every other target. Subsecond wall-clock
time requires luv (non-vanilla mode).

Addresses #12843 (hxcoro with -D lua-vanilla).

Author: jdonaldson

std/haxe/Timer.hx

@@ -173,6 +173,8 @@ class Timer {
 		return untyped __global__.__time_stamp();
 		#elseif python
 		return Sys.cpuTime();
+		#elseif lua
+		return lua.Os.clock();
 		#elseif sys
 		return Sys.time();
 		#else

std/lua/_std/Sys.hx

@@ -46,7 +46,7 @@ class Sys {
 	public static inline function programPath():String return haxe.io.Path.join([getCwd(), Lua.arg[0]]);
 	public static function getCwd():String return notImplemented();
 	public static function setCwd(s:String):Void notImplemented();
-	public static function getEnv(s:String):Null<String> return notImplemented();
+	public static inline function getEnv(s:String):Null<String> return lua.Os.getenv(s);
 	public static function putEnv(s:String, v:Null<String>):Void notImplemented();
 	public static inline function setTimeLocale(loc:String):Bool return lua.Os.setlocale(loc) != null;
 	public static function sleep(seconds:Float):Void notImplemented();