Mark locale feature as experimental

[joelim-work]

The locale feature was requested in #323, and while it was implemented to a reasonable degree in #1818, I still have some slight concerns about it:

• The feature was developed with mainly sorting Chinese text in mind, however there are many other languages in existence and there is little to no testing for those.
• The addition of the collate library increases the size of the compiled binary from around 4MB to 5.6MB (an increase of 40%). This concern has been raised in File size of `lf` executable increased #1836.
• There is a possibility that locale-aware sorting could be lumped with all the other custom sorting requests as part of Sort by custom method #618, making the current implementation redundant.

In any case, unless there are issues raised from other users, I think it is unlikely that I will make any changes to the locale feature in future. However I am still marking it as experimental just in case.