scalar: include in standard Git build & installation

scalar: include in standard Git build & installation

Move 'scalar' out of 'contrib/' and into the root of the Git tree. The goal
of this change is to build 'scalar' as part of the standard Git build &
install processes.

This patch includes both the physical move of Scalar's files out of
'contrib/' ('scalar.c', 'scalar.txt', and 't9xxx-scalar.sh'), and the
changes to the build definitions in 'Makefile' and 'CMakelists.txt' to
accommodate the new program.

At a high level, Scalar is built so that:
- there is a 'scalar-objs' target (similar to those created in 029bac01a8
  (Makefile: add {program,xdiff,test,git,fuzz}-objs & objects targets,
  2021-02-23)) for debugging purposes.
- it appears in the root of the install directory (rather than the
  gitexecdir).
- it is included in the 'bin-wrappers/' directory for use in tests.
- it receives a platform-specific executable suffix (e.g., '.exe'), if
  applicable.
- 'scalar.txt' is installed as 'man1' documentation.
- the 'clean' target removes the 'scalar' executable.

Additionally, update the root level '.gitignore' file to ignore the Scalar
executable.

Signed-off-by: Victoria Dye <vdye@github.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>