feat: add itemBorderRadius and wrap itemBuilder with IgnorePointer

[coder-with-a-bushido]

Description

Fixes https://github.com/AbdulRahmanAlHamali/flutter_typeahead/issues/585 Fixes https://github.com/AbdulRahmanAlHamali/flutter_typeahead/issues/586

This PR does 2 things:

1. Adds a new property itemBorderRadius which will be applied on the default itemBuilder that is wrapped on the itemBuilder passed by the user.
2. Wraps the itemBuilder passed by the user with an IgnorePointer so that the interaction behaviour (ex. mouse hover) of the wrapping default itemBuilder is not affected. This shouldn't really change much for the user as any onTap or onPress event from the passed itemBuilder is being ignored already due to the default itemBuilder.

Checklist

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• [x] All code passes analysis with no errors or warnings
• [x] Changes work for both Material and Cupertino
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• [x] All existing tests pass
• [x] Example project still compiles and runs
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