Add cmake install steps

[uilianries]

Hello!

Conan is packaging your project here: conan-io/conan-center-index#22891 to be distributed as a public package. As we using CMake to execute most of step, we found the install step missing.

This PR adds a new configuration to install headers and libraries when executing cmake --build build --target install:

$ cmake -S . -B build -DCMAKE_BUILD_TYPE=Debug -DCMAKE_INSTALL_PREFIX=/tmp/install 
...   
$ cmake --build build --target install                                                                                                                                                                                                     10:28:18
[ 11%] Built target ada-idna
[ 22%] Built target punycode_tests
[ 33%] Built target to_ascii_tests
[ 44%] Built target mapping_tests
[ 55%] Built target to_unicode_tests
[ 66%] Built target simdjson
[ 77%] Built target wpt_tests
[ 83%] Built target singleheader-files
[100%] Built target demo
Install the project...
-- Install configuration: "Debug"
-- Installing: /tmp/install/lib/libada-idna.a
-- Installing: /tmp/install/include/idna.h
-- Installing: /tmp/install/include/ada
-- Installing: /tmp/install/include/ada/idna
-- Installing: /tmp/install/include/ada/idna/normalization.h
-- Installing: /tmp/install/include/ada/idna/mapping.h
-- Installing: /tmp/install/include/ada/idna/punycode.h
-- Installing: /tmp/install/include/ada/idna/validity.h
-- Installing: /tmp/install/include/ada/idna/to_ascii.h
-- Installing: /tmp/install/include/ada/idna/unicode_transcoding.h
-- Installing: /tmp/install/include/ada/idna/to_unicode.h

[lemire]

Thanks.