fix: handle os.ErrExist on subsequent reruns

[zalgonoise]

This PR fixes an issue reported by other users a while ago on #21, which pointed out that running the same executable / script several times (e.g. when changing or updating the same diagram) results in os.ErrExist errors considering that the go-diagrams and inner assets folders already existed.

This is a known error returned in os.MkDir and os.MkDirAll that can be checked with an (optionally in-line) errors.Is call.

These changes are applied to the diagram and cmd/gen package, where (black-box) tests were added to the diagram package, and the assets regenerated (with go run ./cmd/gen).

This PR also updates all dependencies to their latest versions; removes the usage of ioutil (as a deprecated package, if favor of os functions); and regenerates all assets. The later is an important step to mention since it seems these were generated in a different order in some symbols. I am unsure of the reason why (didn't look into that) however I can also remove the assets folder commit in particular, if you wish to skip changes to that package. I believe it's always a good idea to regen on changes, but there is a context to it too.

There are other details that I see potential in improvement, and I wish to contribute to this repository, but all in their due PRs and topics :D these changes should only comprehend the issue for the existing files when re-rendering a diagram.

[zalgonoise]

I see that other users have already posted similar (and simpler) PRs for this issue. I will fork the repository and continue working on it for my own requirements :) You can ignore this PR, or re-open it / mention me if you'd like to apply a patch to your repository with any of the content