fix(animation): typescript interface has correct return value for progress methods

[liamdebeasi]

Pull request checklist

Please check if your PR fulfills the following requirements:

• Tests for the changes have been added (for bug fixes / features)
• Docs have been reviewed and added / updated if needed (for bug fixes / features)
• Build (npm run build) was run locally and any changes were pushed
• Lint (npm run lint) has passed locally and any fixes were made for failures

Pull request type

Please check the type of change your PR introduces:

• Bugfix
• Feature
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no api changes)
• Build related changes
• Documentation content changes
• Other (please describe):

What is the current behavior?

TypeScript interface says progressStart, progressStep, and progressEnd methods return void, but they actually return Animation: https://github.com/ionic-team/ionic-framework/blob/master/core/src/utils/animation/animation.ts#L661-L740

ionic-team/ionic-docs#1930

What is the new behavior?

• TypeScript interface has correct return value now

Does this introduce a breaking change?

• Yes
• No

Other information