Ensure column string is symbol before constructing expression for col_vals_*() functions

[yjunechoe]

Resolves #530

This PR preserves columns as symbols in the internal tbl_val_comparison() function.

Instead of creating an expression out of this string:

"column name < 1000"

We now immediately evaluate to the correct expression, using call():

`column name` < 1000

Reprex:

agent <- mtcars |> 
  dplyr::rename(`disp units` = disp) |>
  create_agent() |> 
  col_vals_lt(`disp units`, 1000) |>
  interrogate()
#> 
#> ── Interrogation Started - there is a single validation step ──────────────────
#> ✔ Step 1: OK.
#> 
#> ── Interrogation Completed ────────────────────────────────────────────────────
get_agent_report(agent, display_table = FALSE)
#> # A tibble: 1 × 14
#>       i type       columns values precon active eval  units n_pass f_pass W    
#>   <int> <chr>      <chr>   <chr>  <chr>  <lgl>  <chr> <dbl>  <dbl>  <dbl> <lgl>
#> 1     1 col_vals_… disp u… 1000   <NA>   TRUE   OK       32     32      1 NA   
#> # ℹ 3 more variables: S <lgl>, N <lgl>, extract <int>

[rich-iannone]

LGTM!