vsexprtools: add missing atan2 #67

class ExprOp(ExprOpBase, CustomEnum):
    # Not Implemented in akarin or std
    PI = f"{pi}", 0
    SGN = "dup 0 > x 0 < -", 1
    NEG = "-1 *", 1
    TAN = "dup sin swap cos /", 1

   ...

    @classmethod
    def atan(cls, c: str = "x", n: int = 5) -> ExprList:
        # Approximation using Taylor series
        n = max(2, n)

        expr = ExprList([c, "dup", "var!"])

        for i in range(1, n):
            expr.append("var@", 2 * i + 1, ExprOp.POW, 2 * i + 1, ExprOp.DIV, ExprOp.SUB if i % 2 else ExprOp.ADD)

        return expr

    @classmethod
    def asin(cls, c: str = "x", n: int = 5) -> ExprList:
        return cls.atan(
            str(ExprList([c, ExprOp.DUP, ExprOp.DUP, ExprOp.MUL, 1, ExprOp.SWAP, ExprOp.SUB, ExprOp.SQRT, ExprOp.DIV])), n
        )

    @classmethod
    def acos(cls, c: str = "x", n: int = 5) -> ExprList:
        return ExprList([cls.PI, 2, ExprOp.DIV, cls.asin(c, n), ExprOp.SUB])