Add encodeLE/decodeLE, fixing #19

[hasufell]

TODO:

• make sure filepath uses the internal API to avoid deprecation warnings getting triggered (Adjust for the encodeFS/decodeFS deprecation in os-string filepath#232)

[hasufell]

• added decodeLE/encodeLE that uses getLocaleEncoding
• deprecate decodeFS/encodeFS (which uses getFileSystemEncoding)
• point users to the filepath package to use it
• keep it in the internal API anyway
• also update the haddocks to make the distinction clearer

@Bodigrim