fix(error-overlay): improve a11y, minor refactors (#62723)
### What?
- Use `button` instead of `span` to allow keyboard navigation
- match tabindex with other items in the overlay to include it in the
natural focus sequence
- the style is kept, but we might need a better contrasting icon for the
dark background
- use same file conventions with existing files
- match file casing
- move icons to the `icons` folder
- Improve PseudoHtml internal docs
- use code guards ``` for code snippets
- drop the `any` type (long-term, we really should disallow `any` types)
Closes NEXT-2657
