Add trigger_changevalue from svencoop

The FGD content to help explain what it should do.

// Change any entity keyvalue during runtime, allows set, basic arithmetic operations and bitwise logical operations (useful for flags)
@PointClass base(Targetname) color(90 215 60) = trigger_changevalue : "Trigger Change Value"
[
    spawnflags(Flags) =
    [

        // For Vectors:
        1 : "Don't use X" : 0
        2 : "Don't use Y" : 0
        4 : "Don't use Z" : 0

        32 : "Invert target value" : 0
        64 : "Invert source value" : 0

    ]
    target(target_destination) : "Destination entity"
    m_iszValueName(string) : "Destination key"
    m_iszNewValue(string) : "Source value"
    m_iszValueType(choices) : "Action" :  0 =
    [
        0 : "Replace (= source)"
        1 : "Add (= old + source)"
        2 : "Mul (= old * source)"
        3 : "Sub (= old - source)"
        4 : "Div (= old / source)"
        16 : "Pow (= old ^ source)"
        12 : "Mod (= old % source)"
        5 : "AND (= old & source)"
        6 : "OR (= old | source)"
        13 : "XOR (= old ^ source)"
        7 : "NAND (= !(old & source))"
        8 : "NOR (= !(old | source))"
        14 : "NXOR (= !(old ^ source))"
        11 : "Append (String concatenation)"
        17 : "Sin (= sin(source))"
        18 : "Cos (= cos(source))"
        19 : "Tan (= tan(source))"
        23 : "Cot (= cot(source))"
        20 : "Arcsin (= arcsin(source))"
        21 : "Arccos (= arccos(source))"
        22 : "Arctan (= arctan(source))"
        24 : "Arccot (= arccot(source))"
    ]
    m_trigonometricBehaviour(choices) : "Trigonometric funcs. I/O" : 0 =
    [
        0 : "Degrees in (out for arc.)"
        1 : "Radian measure in (out for arc.)"
    ]
    m_iAppendSpaces(integer) : "Append spaces (for strings)" : 0
    message(target_destination) : "Trigger after action"
]

Start by locating existing trigger entities and the FGD definitions in the MasterSwordRebirth source, then compare their implementation and key-value handling with the supplied trigger_changevalue definition. Use the listed actions, flags, trigonometric modes, and trigger target as the behavior checklist; done means the entity is available with those fields and operations.