Add G-computation correction for emmeans, contrasts argument

Adds emmeans_gcomp_vars to mmrm_control(). When set, emmeans() returns the average treatment effect with standard errors that account for covariate variability across subjects. Default is NULL, when NULL, mmrm runs identically as before. Related to #560, though the counterfactual path does not support variance correction and has 50x runtime overhead only addressable through emmeans changes. mmrm() and fit_mmrm() now accept a contrasts argument for specifying contrast matrices or functions, matching lm(). When a contrast matrix includes levels not present in the fitting data, those levels are preserved and marked as aliased, enabling prediction on new data