Attempt to handle the "completely contained within" timezone rule (#629)

Fixes #627

This does two things:

- It starts including the `backzone` file, which fixes things like
Chuuk/Truk
- I'm including it unconditionally in zoneinfo_rs. I could instead
include it conditionally in datagen.rs as an additional thing we tack
on. Thoughts?
- It has hardcoded overrides for the three packrat timezones. These were
verified from
https://github.com/unicode-org/cldr/blob/main/common/bcp47/timezone.xml.

Author: Manishearth

Cargo.lock

@@ -25,6 +25,8 @@ exclude = [
     "CONTRIBUTING.md",
     "cliff.toml",
     "tests/data/zoneinfo64.res",
+    # Under a different license
+    "provider/src/tzdb/cldr-timezone.xml",
 ]
 
 [workspace.dependencies]

Cargo.toml

@@ -73,3 +73,6 @@ databake = { workspace = true, optional = true, features = ["derive"]}
 yoke = { version = "0.8.0", features = ["derive"], optional = true }
 serde_json = { version = "1.0.145", optional = true }
 
+[dev-dependencies]
+serde-xml-rs = "0.8.1"
+serde = { version = "1.0.225" , features = ["derive"] }