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

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

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

@@ -46,14 +46,14 @@ 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();
 	public static inline function stderr():haxe.io.Output return @:privateAccess new FileOutput(Io.stderr);
 	public static inline function stdin():haxe.io.Input return @:privateAccess new FileInput(Io.stdin);
 	public static inline function stdout():haxe.io.Output return @:privateAccess new FileOutput(Io.stdout);
-	public static function time():Float return notImplemented();
+	public static inline function time():Float return lua.Os.time();
 }
 
 #else