[v10.x] build: expose napi_build_version variable

[gabrielschulhof]

Expose napi_build_version to allow node-gyp to make it
available for building native addons.

Fixes: nodejs/node-gyp#1745
Refs: nodejs/abi-stable-node#371
PR-URL: #27835
Reviewed-By: @bnoordhuis
Reviewed-By: @addaleax
Reviewed-By: @gabrielschulhof
Reviewed-By: @lpinca
Reviewed-By: @richardlau
Reviewed-By: @mhdawson

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• documentation is changed or added
• commit message follows commit guidelines