zig init: allow specifying project path

[fel1x-developer]

This PR allows zig init to initialize project when path is specified.

Usage:

$ zig init # path not provided, initialize in the current directory
$ zig init foo # create dir named foo if it does not exists, then initialize under ./foo
$ zig init foo/bar # create dir tree foo/bar if it does not exists, then initialize under ./foo/bar

When specified path already exists as non-dir, the cli throws fatal()

Closes: #24290