Computes a statistic based on the Lagrange Multiplier (LM) test of Engle (1982) for
autoregressive conditional heteroscedasticity (ARCH). The statistic returned is
the \(R^2\) value of an autoregressive model of order `lags`

applied
to \(x^2\).

`arch_stat(x, lags = 12, demean = TRUE)`

- x
a univariate time series

- lags
Number of lags to use in the test

- demean
Should data have mean removed before test applied?

A numeric value.