Source-loader: Fix invalid call to CSF sanitize

[zhyd1997]

Issue: #12208

What I did

The error was reported by sanitize function, its parameter only accepts the string type:
https://github.com/ComponentDriven/csf/blob/5654bca6cd222684b0128ee4835258d990d3b526/src/index.ts#L8

We import it to ts file in development mode but use it in js file in production mode, so it misses the type restrict.

Here I add an additional type restrict in js style, which uses typeof to determine if its parameter is a string type.

How to test

• Is this testable with Jest or Chromatic screenshots?
• Does this need a new example in the kitchen sink apps?
• Does this need an update to the documentation?

If your answer is yes to any of these, please make sure to include it in your PR.

[shilman]

Thanks so much for your contribution! Would love to get this tested, etc, but in the interest of expediency I'm merging as is.