Add Windows CI badge to README build matrix

[steven-varga]

The README build matrix table shows Ubuntu 22.04 and 24.04 badges for GCC and Clang, but is missing the Windows / MSVC badge that was added in #112.

Task:

• Add an MSVC column to the build matrix table
• Add a Windows row with the MSVC badge
• Badge URL: https://vargalabs.github.io/h5cpp/badges/windows-latest-msvc.svg

Current table:

| OS / Compiler | GCC 13 | GCC 14 | GCC 15 | Clang 17 | Clang 18 | Clang 19 | Clang 20 |

Expected table:

| OS / Compiler | GCC 13 | GCC 14 | GCC 15 | Clang 17 | Clang 18 | Clang 19 | Clang 20 | MSVC |