feat: zig_code_path quality-of-life improvements

[polvalente]

When I first tried zig_code_path, I tried using Path.join in the use opts.
This raised and I settled for a relative path (which I realize is the expected
code path).

Even if this change is not accepted, we can raise in a more informative way
in the new if is_binary(path) conditional.

Also, the put_attribute call makes it so that the code recompiles whenever
the zig source changes.

[polvalente]

There's something I didn't find an easy fix for as well: If you pass zig_code_path, Zigler ends up creating separate .{module}.zig files, and all "go to file" links end up taking to that file.

This in turn were, multiple times, the cause for me having to apply a fix again because I applied it to the separate file instead of my source one.