Add renovate.json

[cakebaker]

I'm not sure whether this is the correct way to enable renovate for this repository.

[codspeed-hq]

Merging this PR will not alter performance

✅ 10 untouched benchmarks
⏩ 17 skipped benchmarks¹

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_{Comparing cakebaker:enable_renovate (9b42417) with main (670669e)}

Footnotes

1. 17 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

[lhecker]

I'm not familiar with renovate, so I'll defer this to @sylvestre.

[cakebaker]

Hm, it looks like it didn't enable renovate as no PRs were generated :|