Support rearguard and vanguard parsing in `zoneinfo_rs`

[nekevss]

Currently, we only support -- by default -- vanguard parsing but not rearguard

It would be nice to provide a configuration option to switch between the two. An example of vanguard/rearguard can be found with Europe/Dublin.

[nekevss]

Worth noting, this would probably require some level of preparsing functionality to zoneinfo_rs

But it can also be supported by directly using AWK.