fixes #1760

• I have read the Contribution Guidelines
• I have commented on the issue above and discussed the intended changes
• A maintainer has signed off on the changes and the issue was assigned to me
• All newly added code is adequately covered by tests
• All existing tests are still running without errors
• The documentation was modified to reflect the changes OR no documentation changes are required.

Changes

Placeholder Styling: Customize the appearance of placeholder text (defaults to grey)

Behavior: Placeholder automatically disappears when user starts typing.

Added Placeholderand PlaceholderStyle properties to TextPrompt<T>
Added tests.

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