Add `print` method for `DeltaMethodForRatio`

[hoxo-m]

Currently, the DeltaMethodForRatio R6 class lacks a custom print method, so it defaults to the generic R6 output. Adding a print method would improve usability by providing a more informative and user-friendly summary.

library(deltatest)
library(dplyr)

n_user <- 100

set.seed(314)
df <- deltatest::generate_dummy_data(n_user) |> 
  group_by(user_id, group) |> 
  summarise(click = sum(metric), pageview = n(), .groups = "drop")

DeltaMethodForRatio$new(df$click, df$pageview)

<DeltaMethodForRatio>
  Public:
    clone: function (deep = FALSE) 
    compute_confidence_interval: function (mean, standard_error, alternative = c("two.sided", 
    compute_expected_value: function (mean1, mean2, var2, cov = 0, bias_correction = FALSE) 
    compute_variance: function (mean1, mean2, var1, var2, cov = 0) 
    get_confidence_interval: function (alternative = c("two.sided", "less", "greater"), conf_level = 0.95) 
    get_expected_value: function () 
    get_info: function (alternative = c("two.sided", "less", "greater"), conf_level = 0.95) 
    get_squared_standard_error: function () 
    get_standard_error: function () 
    get_variance: function () 
    initialize: function (numerator, denominator, bias_correction = FALSE) 
  Private:
    expected_value: 0.215712383488682
    n: 1502
    size: 100
    squared_standard_error: 0.000327578645294461
    standard_error: 0.018099133827188
    variance: 0.0327578645294461
    x: 324