feat(toast): add shadow part for cancel button

[luisbytes]

Issue number: resolves #27920

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What is the current behavior?

It is not possible to style the buttons with the cancel role in a toast since it uses shadow DOM and there are no exposed parts or CSS variables for the cancel buttons.

What is the new behavior?

• Exposes the following part of a toast: button with rol cancel
• Update the e2e toast test to check if the button-cancel part is exposed

Does this introduce a breaking change?

• Yes
• No

Change the css shadow part for all buttons added to a toast with the cancel role from 'button' to 'button-cancel'.

Other information

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[liamdebeasi]

Thanks for the PR! We appreciate you contributing to Ionic. I discussed this with the team, and we think this is a good feature to add. Our next step is to do a design document to make sure we are implementing this feature in a way that addresses the original use case.

Once that is done, someone from the team will review this PR and get it ready to merge. Let me know if you have any questions.

[liamdebeasi]

Overall this looks good! A few small changes.

[liamdebeasi]

Can you also rebase this off feature-7.3? This is the next minor release we are targeting.

[luisbytes]

Can you also rebase this off feature-7.3? This is the next minor release we are targeting.

Yes, I'll get on it

[luisbytes]

@liamdebeasi Hello, I'm ready with the changes you suggested. I failed to do the rebase.

[liamdebeasi]

Great work! I can take care of targeting the feature-7.3 branch.

[liamdebeasi]

Merged. Thank you very much!

[luisbytes]

@liamdebeasi Thank you, I hope to contribute more to the community.