Add complexity param

[camillobruni]

Depends on params.mjs changes in #469

Work in progress proposal for adding a variable complexity param.

• complexity === 1.0: workloads run in default configuration
• complexity > 1.0: ideally workloads run proportionally slower
• complexity < 1.0: ideally workloads run proportionally fast

Background: We're occasionally seeing issues with gc timing / optimisations heuristics with the current workloads. If we can dynamically change the duration / complexity of workloads it's a bit easier to assess how good these heuristics are and how much we're potentially overfitting.

Current Limitations:

• DeveloperMode UI cannot reload tests.mjs causing stale BenchmarkTestStep titles
• Only TodoMVC are currently scaling the workload size (other workloads are depend on whether we're fine with the general idea here)

[bgrins]

Without looking closely at the code change, I think this is a neat idea and I could imagine us increasing the complexity of charts, text editing, etc if we move forward with it. Will be curious for feedback from @julienw.

[camillobruni]

• Only create new Params object if there are URL search params
• Added table to highlight non-standard params if there are is a default to the defaultParams object

[camillobruni]

I've now changed the complexity slider to have an exponential scale which makes it a bit more comfortable to use.

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[camillobruni]

We should probably log all non-standard params to the json file. let me do that in a separate PR. In our infra we use the json file to populate perf data.

[camillobruni]

This is ready for review again.