allow customizing decrease delay of rgb heatmap (#11322)
* allow customizing decrease delay of rgb heatmap
* rename rgb typing heatmap decrease delay variable
* address review comments
* nix-shell: add clang-tools required for formatting the C code
* heatmap: use real timer to track decrement rate
* heatmap: fix ifndef var name typo
* heatmap: add docs
* Update docs/feature_rgb_matrix.md
Co-authored-by: Drashna Jaelre <drashna@live.com>
Co-authored-by: Drashna Jaelre <drashna@live.com>