Feature Request: Discard tabs based on usage

[arjpar]

My request is similar to #180 , but would specifically replicate functionality in Firefox that is coming to Chromium and currently available in a flag. In Chromium, you can enable flags that create improvements to discarding tabs based on usage.

Specifically this flag:
"Enable the multi-state option for Memory Saver Mode.
When enabled, Memory Saver can take one of three options: enabled with a heuristic mode [...]"

My feature request would propose a heuristic based mode to determine which tabs to discard, not only a timed-mode which would be a separate option.

The relevant code in Chromium might be [here](https://source.chromium.org/chromium/chromium/src/+/main:chrome/browser/performance_manager/metrics/metrics_provider_desktop.cc;l=95;bpv=1;bpt=1?q="heuristic mode"&sq=&ss=chromium%2Fchromium%2Fsrc). I'm not sure if this FR would even be possible in Firefox, so it would require some investigation.