Implement update check functionality with version comparison and archive listing

- Added `check.rs` to handle package update checks, including logic for determining available updates from remote git repositories and archive listings.
- Introduced `versions.rs` to manage version patterns, comparison logic, and extraction of candidate versions from git refs and archive listings.
- Implemented `hex.rs` for encoding byte arrays to lowercase hexadecimal strings.

src/hex.rs

@@ -0,0 +1,9 @@
+pub(crate) fn encode_lower(bytes: impl AsRef<[u8]>) -> String {
+    let bytes = bytes.as_ref();
+    let mut out = String::with_capacity(bytes.len() * 2);
+    for byte in bytes {
+        use std::fmt::Write as _;
+        let _ = write!(&mut out, "{byte:02x}");
+    }
+    out
+}