feat(item): add styles for disabled state in ionic theme (#29583)

Issue number: internal

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## What is the new behavior?
- Added scss for the disabled state of `ion-item` for the Ionic theme
- Added disabled elements to test page

## Does this introduce a breaking change?
- [ ] Yes
- [x] No

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Co-authored-by: Brandy Carney <brandyscarney@users.noreply.github.com>
Co-authored-by: ionitron <hi@ionicframework.com>
Co-authored-by: Brandy Carney <brandyscarney@gmail.com>