Compute effect sizes of path coefficients in SEM with R

Question

I am currently using the lavaan package in R for structural equation models. I would like to compute the effect sizes (i.e., partial-eta-squared) for each of my path coefficient. Is there already a package that does this?

For instance, how can I compute the effect size of the c, a and b regression coefficients?

set.seed(1234)
X <- rnorm(100)
M <- 0.5*X + rnorm(100)
Y <- 0.7*M + rnorm(100)
Data <- data.frame(X = X, Y = Y, M = M)
model <- ' # direct effect
             Y ~ c*X
           # mediator
             M ~ a*X
             Y ~ b*M
           # indirect effect (a*b)
             ab := a*b
           # total effect
             total := c + (a*b)
         '
fit <- sem(model, data = Data)
summary(fit)

Ideally the method should also work when building models based on latent variables.