Move getStablePath method into the "url" package

[anton-vlasenko]

What problem does this address?

Both createPreloadingMiddleware and createMenuPreloadingMiddleware functions need to be able to call the getStablePath method.
As of now, we have some duplicate code (2 getStablePath methods defined).
See this and that for reference.
More context can be found here: https://github.com/WordPress/gutenberg/pull/35402/files#r728631767

What is your proposed solution?

• Move getStablePath into the url package (and possible rename it to normalizeQueryString).
• Import normalizeQueryString function from the url package and use it for createPreloadingMiddleware and createMenuPreloadingMiddleware instead of getStablePath
• Delete getStablePath method (check if it used elsewhere in the project first).