projects: generate from a single template

The three projects (VC10, VC11, VC12) are identical except 5 repeated
strings in them. They also require running `generate.bat` before use,
to populate source files. Reduce the 3 almost identical projects to
a single template project and populate the repeated strings also via
`generate.bat`. This reduces the maintenance burden to a single copy of
the project files. Also saving 10000 LOCs.

Closes curl#18396

Author: vszakats