Support Svelte

**Is your feature request related to a problem? Please describe.**
I use @storybook/addon-svelte-csf, but they aren't linted like other stories.

**Describe the solution you'd like**
It should support linting `.svelte` files _when `svelte-eslint-parser` is installed_.

**Describe alternatives you've considered**
Don't.

**Additional context**
It's still "incubating," I guess?
