feat(material-experimental/mdc-button): support custom leading/trailing icons

[mmalerba]

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[mmalerba]

It occurs to me that an alternate solution to this could be to just document adding ngProjectAs="mat-icon" to the content. I wonder if <div ngProjectAs="mat-icon" iconPositionEnd> would work though. Or maybe people would need to do <div ngProjectAs="mat-icon[iconPositionEnd]">?

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