Formatting: Use is_scalar() in sanitize_key().

This is a follow-up to [52292] which introduced `is_string()` to check the given key is a string to be sanitized, else the key is set to an empty string. 

`sanitize_key()` is clearly identified (in the documentation) to only work with ''string'' keys. However, it had a bug in it that allowed non-strings to pass through it:
* A non-scalar "key" would throw a PHP Warning (which was resolved in [52292]. 
* A non-string scalar "key" was handled by the PHP native `strtolower()` which converted it into a string.

While `is_string()` is valid, non-string scalar types passed as the key to be sanitized were being set to an empty string. Given that `strtolower()` handles these without error or deprecation as of PHP 8.1, `is_scalar()` protects the website from issues while retaining the past behavior of converting integer keys (for example) into a string.

Changes include:
* Using `is_scalar()` instead of `is_string()`
* Refactor for readability and less code
* More tests

Please note, this does not change the behavior of the function, nor redefine it to now accept non-string scalars.

References:
* https://developer.wordpress.org/reference/functions/sanitize_key/
* https://www.php.net/manual/en/function.strtolower.php

Follow-up [52292].

Props wppunk, hellofromTonya, costdev, jrf.
Fixes #54160.
Built from https://develop.svn.wordpress.org/trunk@52370


git-svn-id: http://core.svn.wordpress.org/trunk@51962 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/version.php

@@ -16,7 +16,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.9-beta2-52369';
+$wp_version = '5.9-beta2-52370';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.