common.mk: Add file

This is the first in a series of attempts to clean up jw-pkg's makefile
fragments make/*.mk. It collects redundantly defined workspace and project
variables into one shared common.mk.

Variables needed in both the workspace directory and the project
directories are defined separately in the two contexts: Q in defs.mk and
projects-dir-minimal.mk, INTERACTIVE in topdir.mk and projects-dir.mk,
BROWSER and JWB_SCRIPT_DIR in defs.mk/platform.mk and projects-dir.mk, and
LIST_VCS_FILES and LIST_VCS_FILES_SH wrapping the same scm.sh ls-files call
under different names. PKG_MANAGER is defined in topdir.mk and
projects-dir.mk, and JW_PKG_PY_PROJECTS is a prefix only used to build two
other variables in projects-dir.mk.

This commit adds make/common.mk, included from defs.mk (and hence from
every project directory) and from projects-dir.mk (the workspace), and
collects the shared variables there:

- Q, INTERACTIVE, BROWSER, JWB_SCRIPT_DIR: one ?= definition each
- LIST_VCS_FILES: one definition; LIST_VCS_FILES_SH is dropped and the
  list-files target uses the shared name
- PKG_MANAGER moves to projects+project.mk, the fragment shared by the
  workspace and the topdir
- projects-dir-minimal.mk keeps its own Q, because the bootstrap recipes
  run before common.mk is parsed
- JW_PKG_PY_PROJECTS is inlined into JW_PKG_PY_BUILD and
  JW_PKG_PY_REQUIRED_OS_PKG

Signed-off-by: Jan Lindemann <jan@janware.com>
Assisted-by: unsloth/Qwen3.8-27B-GGUF:Q4_K_M with pi.dev v0.87.0

make/projects-dir-minimal.mk

@@ -18,6 +18,8 @@ PROJECTS_MAKEFILE_NAME      := $(firstword $(MAKEFILE_LIST))
 -include local.mk
 
 JWBDIR                      ?= jw-pkg
+# The bootstrap recipes below run before jw-pkg's common.mk is
+# parsed, so Q must be defined here; a no-op in the steady state.
 Q                           ?= @
 
 ifeq ($(origin PROJECTS_DIR_REMOTE_BASE),undefined)