fix: Forbid reuse of new columns created in summarize()

[krlmlr]

Closes #72.

With this PR:

options(conflicts.policy = list(warn = FALSE))
library(duckplyr)

Sys.setenv(DUCKPLYR_FORCE = TRUE)

data <- tibble(a = c(1L, 1:2), b = 1:3, c = 4:6)
data
#> # A tibble: 3 × 3
#>       a     b     c
#>   <int> <int> <int>
#> 1     1     1     4
#> 2     1     2     5
#> 3     2     3     6

data |>
  as_duckplyr_df() |>
  summarize(
    .by = a,
    b = sum(b),
    c = sum(b * c),
  )
#> Error in `FUN()`:
#> ! Can't reuse summary variable `b`.

^{Created on 2024-03-07 with reprex v2.1.0}