refactor(winreg): reorganize Windows registry utilities into extra module

Move winreg_extra functionality into new extra/winreg module and restructure project layout
Add Windows icon handling utilities in extra/windows module
Update dependencies and clean up unused wincmd module

Cargo.lock

@@ -739,6 +739,7 @@ dependencies = [
  "regex",
  "thiserror",
  "uuid",
+ "widestring",
  "windows-sys 0.60.2",
  "winreg",
 ]
@@ -764,6 +765,12 @@ dependencies = [
  "wfassoc",
 ]
 
+[[package]]
+name = "widestring"
+version = "1.2.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "72069c3113ab32ab29e5584db3c6ec55d416895e60715417b5b883a357c3e471"
+
 [[package]]
 name = "winapi"
 version = "0.3.9"