glmnet: Require users to set penalty value for poisson_reg()

[hfrick]

closes #757

I'll add the corresponding test to the other glmnet tests for poisson_reg() in extratests.

library(poissonreg)
#> Loading required package: parsnip

# These should both fail in translate()

poisson_reg(penalty = c(0.01, 0.1)) %>% 
  set_engine("glmnet") %>% 
  fit(count ~ (.)^2, data = seniors)
#> Error in `.check_glmnet_penalty_fit()` at parsnip/R/poisson_reg.R:89:4:
#> ! For the glmnet engine, `penalty` must be a single number (or a value of `tune()`).
#> • There are 2 values for `penalty`.
#> • To try multiple values for total regularization, use the tune package.
#> • To predict multiple penalties, use `multi_predict()`

poisson_reg() %>% 
  set_engine("glmnet") %>% 
  fit(count ~ (.)^2, data = seniors)
#> Error in `.check_glmnet_penalty_fit()` at parsnip/R/poisson_reg.R:89:4:
#> ! For the glmnet engine, `penalty` must be a single number (or a value of `tune()`).
#> • There are 0 values for `penalty`.
#> • To try multiple values for total regularization, use the tune package.
#> • To predict multiple penalties, use `multi_predict()`

^{Created on 2023-01-22 with reprex v2.0.2}

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