feat(progress-bar): add transition css variables

[EinfachHans]

closes #21820

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?

Issue Number: #21820

What is the new behavior?

• --progress-transition added
• --buffer-transition added
• both default to old value: transform 150ms linear

Does this introduce a breaking change?

• Yes
• No

[liamdebeasi]

Thanks for the PR! We appreciate the work you put into creating this. We recently added CSS Shadow Parts to ion-progress-bar via #22938 that will allow for customization of the progress bar transition, so I am going to close this PR.

The new feature should be available soon.