feat(input): add the readonly styles for the ionic theme

[brandyscarney]

⚠️ Blocked by https://github.com/ionic-team/ionic-framework/pull/29482 :warning:

Issue number: internal

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What is the current behavior?

There are no custom styles when readonly is added to an input.

What is the new behavior?

• Applies an input-readonly class when the readonly property is true
• Styles the class in the ionic theme to match the design requirements for readonly inputs
• Adds screenshot tests for the readonly style for an unset fill and "outline" fill

Does this introduce a breaking change?

• [ ] Yes
• [x] No

Other information

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Name	Status	Preview	Comments	Updated (UTC)
ionic-framework	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	May 9, 2024 7:40pm