allow ref var with initial value

[MkazemAkhgary]

allow to use ref var with initial or default value.

if (!map.AddOrFind(ref var kvp = (item, slot).AsPair(), false);
{
    // ...
}

equivalent to

var kvp = (item, slot).AsPair();
if (!map.AddOrFind(ref kvp, false))
{
   // ...
}

AsPair is extension method to convert tuple to key value pair.

public static KeyValuePair<TKey, TValue> AsPair<TKey, TValue>(this (TKey key, TValue value) tuple)
{
    return new KeyValuePair<TKey, TValue>(tuple.key, tuple.value);
}

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edited with real example of what I had, AddOrFind method exist here

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some considerations:

• should this syntax be allowed too?

Call(ref var x, y); // x assigns to default(T), var is inferred from method signature if there are no ambiguous overloads.

[jnm2]

Duplicate of #854