`columns` for filters

Given a record or table, produce a list of its columns' names.

Signature

> columns {flags}

input	output
table	list<string>
record	list<string>

Get the columns from the record

> { acronym:PWD, meaning:'Print Working Directory' } | columns
╭───┬─────────╮
│ 0 │ acronym │
│ 1 │ meaning │
╰───┴─────────╯

Get the columns from the table

> [[name,age,grade]; [bill,20,a]] | columns
╭───┬───────╮
│ 0 │ name  │
│ 1 │ age   │
│ 2 │ grade │
╰───┴───────╯

Get the first column from the table

> [[name,age,grade]; [bill,20,a]] | columns | first

Get the second column from the table

> [[name,age,grade]; [bill,20,a]] | columns | select 1

This is a counterpart to values, which produces a list of columns' values.