Columns Shortcode doesn't accept `auto` for column count

**Steps to reproduce:**
Try to set the column count to `auto` using the shortcode parameter `count`:
`[columns count=auto] [/columns]`

**Expected results:**
The resulting CSS style should have a column count of `auto`.

**Actual results:**
The resulting CSS style has a column count of `0`.

**More details:**
Currently, the `count` parameter of the Columns shortcode is of type number, and is cast to an integer when generating the CSS style. However, `auto` is a valid value for the column count in the `columns` and `column-count` CSS properties (see [MDN reference](https://developer.mozilla.org/en-US/docs/Web/CSS/columns#formal_syntax)).

I suggest that the shortcode parameter should accept the `auto` value as well. It provides a simple way to add some reactivity to the columns element by having the column number dynamically determined given the set column with, resulting in more columns on a wider viewport.