topdir.mk: Add target gitignore

Add a target gitignore to topdir.mk, which blasts jw-pkg's default idea of
how .gitignore should look like into $(TOPDIR)/.gitignore. It's not a
prerequisite of any other target, because it's not meant to run
automatically. The idea is to keep some central maintenance of what should
be in there from jw-pkg's perspective, make it convenvient to update, but
leave the final decision to the commiter of any project using the
machinery.

Signed-off-by: Jan Lindemann <jan@janware.com>

conf/templates/gitignore

@@ -0,0 +1,55 @@
+{top}
+*.a
+*.dep.mk
+*.dist
+*.done
+*.jw-secret
+*.jw-secret-file
+*.ldscript
+*.o
+*.old
+*.orig
+*.out
+*.pc
+*.pyc
+*.rep
+*.secret
+*.so
+*.so.*
+*.swp
+*_generated_*
+*tmp*
+*~
+-test-out.txt
+.cache-project.mk
+.exrc
+.feedfs_history
+.gdb_history
+.jw-agit
+.mypy_cache
+.ruff_cache
+__init__.py
+__pycache__
+bin/*.bat
+bin/*.pl
+bin/*.pm
+bin/*.py
+bin/*.sh
+bin/*.tcl
+conftest.py
+core
+dist
+ld-*.conf
+local.conf
+local.mk
+local.mk
+local.yaml
+make/*.pc
+mkspec.sh
+pr-descr*.txt
+pre-local.mk
+pyproject.toml
+pyrightconfig.json
+!*.jw-tmpl
+!*.tmpl
+{end}