Fix CI failures: add vk-bootstrap submodule and fix MSYS2 stormlib

- Add vk-bootstrap as a git submodule (was in extern/ but not tracked,
  causing "vk-bootstrap not found" on all CI jobs)
- Whitelist extern/vk-bootstrap in .gitignore
- Move stormlib to a separate optional install step on both Windows CI
  jobs (mingw-w64-x86_64-stormlib doesn't exist in MSYS2 repos)

.github/workflows/build.yml

@@ -189,9 +189,12 @@ jobs:
             mingw-w64-clang-aarch64-vulkan-loader
             mingw-w64-clang-aarch64-vulkan-headers
             mingw-w64-clang-aarch64-shaderc
-            mingw-w64-clang-aarch64-stormlib
             git
 
+      - name: Install optional packages
+        shell: msys2 {0}
+        run: pacman -S --noconfirm --needed mingw-w64-clang-aarch64-stormlib || true
+
       - name: Configure
         shell: msys2 {0}
         run: cmake -S . -B build -G Ninja -DCMAKE_BUILD_TYPE=Release
@@ -248,10 +251,13 @@ jobs:
             mingw-w64-x86_64-vulkan-loader
             mingw-w64-x86_64-vulkan-headers
             mingw-w64-x86_64-shaderc
-            mingw-w64-x86_64-stormlib
             mingw-w64-x86_64-nsis
             git
 
+      - name: Install optional packages
+        shell: msys2 {0}
+        run: pacman -S --noconfirm --needed mingw-w64-x86_64-stormlib || true
+
       - name: Configure
         shell: msys2 {0}
         run: cmake -S . -B build -G Ninja -DCMAKE_BUILD_TYPE=Release