feat(toast): add part for button cancel

[luisbytes]

Issue number: #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! As I noted in #27921 (comment), we think this is a good feature to add to Ionic. However, Ionic 6 development ended when Ionic 7 shipped earlier this year. As a result, we do not have plans to backport this feature to Ionic 6.

I am going to close this, but let me know if you have any questions.