Add themes weeping-willow, mirthful-willow

[lvdh]

Dear maintainer, this PR contains two suggested delta themes.

Thank you so much for your work on delta!

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dark mode example, weeping-willow in a kitty terminal

light mode example, mirthful-willow in a kitty terminal

[lvdh]

All checks seem stuck at: "Expected — Waiting for status to be reported"
Closed and reopened the PR in an attempt to trigger a new run the checks.

[lvdh]

All checks have passed

[dandavison]

Thanks @lvdh! Very nice additions to the collection.