Returns a one-row data.frame of model-level summaries in the
column convention used by the broom ecosystem
(R2, adj_R2, sigma,
statistic, p_value, df, logLik,
AIC, BIC, deviance, df_residual,
nobs). The statistic and p_value columns
report the omnibus likelihood ratio test of all slopes equal to
zero (the FIML analog of the lm omnibus F-test).