Adds PluginSlug & PluginWpOrgDirectorySlug

[oguzkocer]

Builds on #90.

• The plugin slugs returned from the /plugins endpoint will be in foo/bar format, for example hello-dolly/hello & classic-editor/classic-editor
• The plugin slugs in WordPress.org directory is in foo format: hello-dolly & classic-editor

As explained in #90, there are two different plugin slugs and if we use String type for both, they'll be easy to mix up. The PR introduces PluginSlug & PluginWpOrgDirectorySlug types to address the issue.

Note that we don't use the new type pattern for this, because new type pattern will allow passing the original type as a valid value, making it less effective. I think we might even want to make UserId to be a proper type to avoid this issue - although with UserId it's more of a heads up to developers rather than a guard against a practical issue. So, I could see an argument for using the new type pattern for those cases.

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To Test

• make test-server && make dump-mysql && make backup-wp-content-plugins
• cargo test --test '*' -- --nocapture --test-threads 1

[crazytonyli]

Do you mind adding some docs (similar to your PR description) to these two types?

[oguzkocer]

Not at all, but do you mind if that happens as part of the general documentation effort that's scheduled in the next milestone-ish?

I am not sure about the format yet, so I don't want to add a random one at the moment. However, be assured that everything public will have documentation soon 😅