Conveniently provide active build types to port files

[dg0yt]

Is your feature request related to a problem? Please describe.
To deal with the actually selected build types, port files and maintainer functions need to write:

if(NOT DEFINED VCPKG_BUILD_TYPE OR VCPKG_BUILD_TYPE STREQUAL "debug")
...
endif()
if(NOT DEFINED VCPKG_BUILD_TYPE OR VCPKG_BUILD_TYPE STREQUAL "release")
...
endif()

This is rather long for a common condition and easy to get wrong (DEFINED? Release?).
This is one of the barriers for properly supporting single-config triplets in portfiles.

Proposed solution
Provide a list variable with the active build types.

if("debug" IN_LIST VCPKG_ACTIVE_BUILD_TYPES)
...
endif()
if("release" IN_LIST VCPKG_ACTIVE_BUILD_TYPES)
...
endif()

Describe alternatives you've considered
VCPKG_BUILD_TYPES would be shorter but invites typos due to similarity with VCPKG_BUILD_TYPES.

Discrete boolean variables would allow even shorter conditions, and remove ambiguity about spelling/case:

if(VCPKG_BUILD_DEBUG)
...
endif()
if(VCPKG_BUILD_RELEASE)
...
endif()

[autoantwort]

I strongly prefer VCPKG_BUILD_DEBUG and VCPKG_BUILD_RELEASE :)

[Neumann-A]

related #7733

[dg0yt]

related #7733

That PR includes per-build-type properties which I also considered but didn't want to add to this specific issue. 👍