RFE: add possibility to get output of vcs_tag() without configuring file

Nobody has claimed this yet.

In one of the projects I encountered situation where version.h.in should contain information about not only about git version but also some other stuff.

Right now it is solvable by something like:

```meson
version_array = meson.project_version().split('.')
version_major = version_array[0].to_int()
version_minor = version_array[1].to_int()
version_data = configuration_data()
version_data.set('PROJECT_VERSION_MAJOR', version_major)
version_data.set('PROJECT_VERSION_MINOR', version_minor)
# We don't want to substitute it by noop
version_data.set('PROJECT_VERSION_GIT', '@VCS_TAG@')
version_cc_in = configure_file(
input : 'version.cc.in',
output : 'version.cc.in',
configuration : version_data,
)
version_cc = vcs_tag(
input : version_cc_in,
output : 'version.cc',
)
```

but this is not really nice.

Start by tracing the vcs_tag() and configure_file() entry points in the Meson implementation and review how the shown Meson example passes @VCS_TAG@ between them. Determine the API needed to expose vcs_tag() output without configuring a file, then add coverage for that usage and verify the existing configured-file behavior remains unchanged.

Written by the indexing model from the issue text.