win, build: generate .sln only when necessary

When generating sln, store flags passed to configure. Next time, if
node.sln exists and configure flags match those stored, skip building
.sln files.

Adds projgen vcbuild option to force .sln regeneration.

PR-URL: https://github.com/nodejs/node/pull/21284


Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>