ReleaseNote

Upgrade libiconv to 1.19 and update VS project to VS 2026
- Update libiconv source from 1.18 to 1.19
  - Fixed GB18030 converter bug
  - Added OS/2 special support
  - Fixed --enable-relocatable on native Windows
- Upgrade Visual Studio project to VS 2026
  - Solution version: 17 -> 18
  - Platform toolset: v143 -> v144
- Rename helper scripts from vc2022 to vc2026
  - mingw64_vc2026.cmd now auto-detects VS edition
- Update GitHub Actions workflow

New in 1.19:
* Fixed a bug in the GB18030 converter.
* Special support for OS/2: Encoding names that are results of
  kLIBC nl_langinfo(CODESET) are recognized as aliases.
  (Contributed by KO Myung-Hun.)
* Fixed --enable-relocatable on native Windows.