finishing touches on columns yaml round trip

[yjunechoe]

A small patch in yaml read/write:

• Writing columns is now c("a") instead of c(a) for extra clarity (since the latter can scope an env variable named a). More generally, the strict translation of vars(a) is c("a"), though users can get by with using the more familiar c(a) (and they still get the tidyselect deprecation warning if they accidentally scope an env var)
• Reading is now aware of all tidyselect helpers, instead of just the handful that used to be supported. It now reads e.g., where(is.numeric) as-is, and not as "where(is.numeric)"

Complex expressions can now make the round trip:

agent_pre <- create_agent(~ small_table) |> 
  col_vals_lt(
    columns = where(is.numeric) & starts_with("c"),
    value = 8
  )
yaml_agent_string(agent_pre)
#> type: agent
#> tbl: ~small_table
#> tbl_name: ~small_table
#> label: '[2023-11-06|11:34:30]'
#> lang: en
#> locale: en
#> steps:
#> - col_vals_lt:
#>     columns: where(is.numeric) & starts_with("c")
#>     value: 8.0

agent_yaml <- tempfile()
yaml_write(agent_pre, expanded = FALSE, filename = agent_yaml)
agent_post <- yaml_read_agent(agent_yaml)
yaml_agent_string(agent_post)
#> type: agent
#> tbl: ~small_table
#> tbl_name: ~small_table
#> label: '[2023-11-06|11:34:30]'
#> lang: en
#> locale: en
#> steps:
#> - col_vals_lt:
#>     columns: where(is.numeric) & starts_with("c")
#>     value: 8.0

Preserves old behavior of resolving the expr to column names when expand=TRUE (again, c("c") instead of vars(c)):

agent_pre <- create_agent(~ small_table) |> 
  col_vals_lt(
    columns = where(is.numeric) & starts_with("c"),
    value = 8
  )
yaml_agent_string(agent_pre, expanded = TRUE)
#> type: agent
#> tbl: ~small_table
#> tbl_name: ~small_table
#> label: '[2023-11-06|11:38:46]'
#> lang: en
#> locale: en
#> steps:
#> - col_vals_lt:
#>     columns: c("c")
#>     value: 8.0

agent_yaml <- tempfile()
yaml_write(agent_pre, expanded = TRUE, filename = agent_yaml)
agent_post <- yaml_read_agent(agent_yaml)
yaml_agent_string(agent_post)
#> type: agent
#> tbl: ~small_table
#> tbl_name: ~small_table
#> label: '[2023-11-06|11:38:46]'
#> lang: en
#> locale: en
#> steps:
#> - col_vals_lt:
#>     columns: c("c")
#>     value: 8.0

[rich-iannone]

Looks great!