Add pre_build_script support, similar to post_build_script.
For backwards compatibility with existing zmake.json files, context.config.get("pre_build_script", "") is called instead of context.config["post_build_script"] to avoid errors if "pre_build_script" is undefined.
Expand both pre_build_script and post_build_script contents with os.path.expanduser to expand ~ into the user's home directory.
pre_build_scriptsupport, similar topost_build_script.zmake.jsonfiles,context.config.get("pre_build_script", "")is called instead ofcontext.config["post_build_script"]to avoid errors if"pre_build_script"is undefined.pre_build_scriptandpost_build_scriptcontents withos.path.expanduserto expand~into the user's home directory.