slider value should always be a number

[interim17]

Review time: xmall, < 5 minutes

Problem

Slider value will never be an array since we don't use a range, and we can enforce that by setting range prop to false. @ShrimpCryptid pointed out in NBSV review that the previous type cast would throw an error if it had ever been an array.

• Maintenance

Steps to Verify:

1. Make sure slider still works

[github-actions[bot]]

Coverage report

St.:grey_question:	Category	Percentage	Covered / Total
🟡	Statements	73.24%	602/822
🟡	Branches	68.38%	80/117
🔴	Functions	40.31%	79/196
🟡	Lines	71.7%	537/749

Test suite run success

99 tests passing in 7 suites.

Report generated by 🧪jest coverage report action from aad474e94e157e7e4328e1d9927ea4f4e3a91eed