Remove manual mapping in `FromStr` implementation for `StorySelector` (#3018)

This PR removes the need for writing manual mappings in the `FromStr`
implementation for the `StorySelector` enum used in the storybook CLI.

We are now using the
[`EnumString`](https://docs.rs/strum/0.25.0/strum/derive.EnumString.html)
trait from `strum` to automatically derive snake_cased names for the
enums.

This will cut down on some of the manual work needed to wire up more
stories to the storybook.

Release Notes:

- N/A