Add new `"fillgradient"` colormode

[tpvasconcelos]

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📚 Documentation preview 📚: https://ridgeplot--244.org.readthedocs.build/en/244/

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[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 100.00%. Comparing base (de70328) to head (ec51552). Report is 6 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #244 +/- ## ========================================= Coverage 100.00% 100.00% ========================================= Files 13 13 Lines 449 455 +6 Branches 51 52 +1 ========================================= + Hits 449 455 +6 ``` | [Flag](https://app.codecov.io/gh/tpvasconcelos/ridgeplot/pull/244/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Tomas+Pereira+de+Vasconcelos) | Coverage Δ | | |---|---|---| | [combined-src](https://app.codecov.io/gh/tpvasconcelos/ridgeplot/pull/244/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Tomas+Pereira+de+Vasconcelos) | `99.56% <100.00%> (+<0.01%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Tomas+Pereira+de+Vasconcelos#carryforward-flags-in-the-pull-request-comment) to find out more.

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