Add settings to sort layouts of flexible content alphabetically

[sh-sabbir]

This PR adds an additional option in Flxible Content -> Selection Modal to allow users to sort the layouts alphabetically.
In order to achieve this and make the settings good-looking, minor CSS adjustment is also made.
https://prnt.sc/22a9xgg

[mcaskill]

A while back we implemented a similar flexible content layout sorting mechanism. In our case, layouts can be assigned a position value to ensure layouts before/after others. We mostly used it to ensure certain layouts always appeared first or last.

https://github.com/locomotivemtl/wp-lib-cms/blob/7601919/includes/Modules/ACF/FieldTypes.php#L271-L329