feat(number-input): match readonly variant

[janhassel]

Related #8806

This PR aligns the readonly number input variant with the behaviour and visual apperance of the text input.

Changelog

Changed

• Update behaviour and visual appearance of readOnly variant of the NumberInput

Testing / Reviewing

Ensure no regression unrelated to the following changes occured:

• Has EditOff16 icon
• Transparent input background
• Normal cursor (can select text)
• readonly takes precedence over invalid
• readonly takes precedence over warn
• readonly takes precedence over disabled
• No steppers shown

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