tsc: don't rely on implicit index signature for objects

[brendankenny]

tracking issue for updates needed for the upcoming TypeScript 3.1, which will no longer automatically add [x: string]: any; to any type inferred from an object declaration. This is only a change for js; this was never done in ts.

It was helpful in cases where we were mapping specific small objects to a general grab-bag type (e.g. {type: 'code', snippet: str} and then saying it was of a more general details item type), and we'll have to explicitly declare a few more types to fix those places.

But it means certain errors will now be caught. e.g. in TS≤3.0 in lighthouse-ext-background.js, if you refer to background.getDefaultCategories() the return value will be type checked, but if you do background.getCategoriez() the compiler will think the call is valid and returns an any, because the compiler was automatically adding a [x: string]: any; to the lighthouse-background.js export.

Before I started we had almost 100 errors from this, and fixing them only makes things more correct (even in 3.0), so I thought I'd start early. In many places a single line fix and multiple errors are resolved, so it shouldn't be too bad.

• config files core(tsc): type check config files #5858
• mainthread-work-breakdown/task-groups core(tsc): add explicit index signature in mainthread-work-breakdown #5859
• many places core(tsc): remove more reliance on implicit index signatures #5874
• traceOfTab (and fix optional properties) core(tsc): tighten traceOfTab timing types #5887
• LHError core(tsc): don't use index signature in LHErrors #5896
• load-simulator/simulator.js core(tsc): don't rely on index signatures in load-simulator/simulator.js #5897
• Viewer core(tsc): update to ts 3.1 for new protocol defs #5942
• DetailsItem

[brendankenny]

This is complete with #5942 (though detailsItem still needs work independent of this)