Switch to recommended vcpkg setup

Vcpkg now lives inside a submodule instead of expecting it to be present
on the target system, as recommended by their documentation.

Also fixes #26.

.gitmodules

@@ -13,3 +13,6 @@
 [submodule "External/mINI"]
 	path = External/mINI
 	url = https://github.com/pulzed/mINI.git
+[submodule "External/vcpkg"]
+	path = External/vcpkg
+	url = https://github.com/microsoft/vcpkg.git

CMakePresets.json

@@ -17,7 +17,7 @@
         "DIRECTX_ARCH": "x64",
         "CMAKE_CXX_COMPILER": "cl.exe",
         "CMAKE_TOOLCHAIN_FILE": {
-          "value": "$env{VCPKG_ROOT}/scripts/buildsystems/vcpkg.cmake",
+          "value": "${sourceDir}/External/vcpkg/scripts/buildsystems/vcpkg.cmake",
           "type": "FILEPATH"
         },
         "VCPKG_TARGET_TRIPLET": "x64-windows",