`data_filter`: Add support for loop indices within functions?

Still within #301, I wonder if it would make sense to add support for loop indices within functions for data_filter, @etiennebacher?

library(datawizard)

df1 <- data.frame(
  id = c(1, 2, 3, 1, 3),
  item1 = c(NA, 1, 1, 2, 3),
  item2 = c(NA, 1, 1, 2, 3),
  item3 = c(NA, 1, 1, 2, 3)
)

# Attempt 1
fun <- function(data, id) {
  min.index <- NULL
  for (i in unique(data[[id]])) {
    min.index[i] <- 2
    x <- data_filter(data, item3 == min.index[i])
  }
  x
}
fun(df1, id = "id")
#> Error: Filtering did not work. Please check the syntax of your `filter`
#>   argument.

# Attempt 2, using quotes
fun <- function(data, id) {
  min.index <- NULL
  for (i in unique(data[[id]])) {
    min.index[i] <- 2
    x <- data_filter(data, "item3 == min.index[i]")
  }
  x
}
fun(df1, id = "id")
#> Error: Filtering did not work. Please check the syntax of your `filter`
#>   argument.

# Attempt 3, using curly brackets
fun <- function(data, id) {
  min.index <- NULL
  for (i in unique(data[[id]])) {
    min.index[i] <- 2
    x <- data_filter(data, item3 == min.index[{i}])
  }
  x
}
fun(df1, id = "id")
#> Error: Filtering did not work. Please check the syntax of your `filter`
#>   argument.

# Workaround is to create the index manually first
fun <- function(data, id) {
  min.index <- NULL
  for (i in unique(data[[id]])) {
    min.index[i] <- 2
    index <- which(data$item3 == min.index[i])
    x <- data_filter(data, index)
  }
  x
}
fun(df1, id = "id")
#>   id item1 item2 item3
#> 4  1     2     2     2

^{Created on 2022-11-05 with reprex v2.0.2}

Start at the data_filter entry point and reproduce the three examples in the issue, including the quoted and curly-brace forms. Done means filtering expressions can access loop indices from within a function without requiring the manual which() workaround.