Fallback if IScriptInstanceHelper::Get/Set are not implemented

[samisalreadytaken]

This maintains parity with Valve's implementation.

When a variable is not found in a class, its get/set metamethods are called instead of falling back to the root environment. Since mapbase adds both to all classes regardless of whether they are implemented or not, expected fallbacks would fail.

An additional work to do would be adding a flag on ScriptClassDesc_t to indicate that a class has IScriptInstanceHelper::Get/Set implemented so that only those will have the metamethods created, saving about 10KB of memory (in 32 bit) as well.

::_func3 <- function()
{
	return entindex();
}

// Use CTFPlayer for TF2
CHL2_Player._func2 <- function() { return 1337 }
CHL2_Player._func1 <- function()
{
	printl( _func3() );
	printl( _func2() );
	local fn = function(){}
	// assignment fails but set returns the value
	printl( fn == ( entindex = fn ) );
}

EntIndexToHScript(1)._func1();

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