Update crosstab(): Enhance test selection with Fisher exact test and observed vs. expected values

[LKobilke]

This issue consists of two requests:

1. Auto-detect & run Fisher exact test when needed, showing its result + a notification.
2. Add option to compare observed vs expected values for clearer chi-square explanations to students.

[MarHai]

Re 1: I am not a huge fan of auto detection but agree that it would be a nice addition.

Re 2: This is somewhat possible, already (so we could just add it to the documentation):

chi2 <- WoJ %>% crosstab(reach, employment, add_total = TRUE, percentages = TRUE, chi_square = TRUE) %>% model()
chi2$expected
chi2$observed