`zig init`: allow specifying project directory

[fel1x-developer]

Currently zig init does not accept creating a project other than current directory, regardless the specified directory exists or not.

As a person who also has some experience in npm/uv, I believe that

$ zig init foo # If directory 'foo' already exists, init in the directory. If not, create 'foo' and then init.

will improve developer experience, especially for newcomers from npm/uv backgrounds.