fix(svelte): Supplement missing member types

[fw6]

Add step prop types for Number and Slider controls

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Additional context

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What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

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• If it's a new feature, provide a convincing reason to add it. Ideally, you should open a suggestion issue first and have it approved before working on it.
• Read the Contributing Guidelines.
• Read the Pull Request Guidelines and follow the Commit Convention.
• Check that there isn't already a PR that solves the problem the same way to avoid creating a duplicate.
• Provide a description in this PR that addresses what the PR is solving, or reference the issue that it solves (e.g. fixes #123).
• Ideally, include relevant tests that fail without this PR but pass with it.

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