feat: Add hetushu.com as a supported source

[blueanchovy]

Hetushu.com hosts some Chinese webnovels that aren't available on other supported sources. Currently there's no way to read these through the app.
I'd like to add it as a source, following the same pattern as outlined in contributing.md. Happy to submit a PR if this looks good to maintainers.
Example novel: https://hetushu.com/book/2991/index.html

[anantham]

@blueanchovy Support for hetushu.com was added in 782b95c — includes a full source adapter with watermark stripping. Can you verify that it works for your use case? The example novel you linked (https://hetushu.com/book/2991/index.html) should be loadable now.