Adopt object spread and polyfill Object.assign

[mourner]

Currently we use a custom extend utility function because Object.assign isn't supported in IE11.

However, if we polyfilled Object.assign in IE, we could switch to object spread ({...obj}) syntax across the code base, which provides many benefits:

• An easier to read, more concise syntax.
• When we ditch IE11 support in future and start providing an untranspiled build, we'll get better performance for free because native spreads are faster than Object.assign.
• Strictly typing extend and all code that uses it in Flow is really hard, which I discovered while working on #8281. It's much easier with spreads because it syntactically indicates the full shape of an object from the start — no type conversions/refinements need to happen to transform an object through multiple extensions.

[asheemmamoowala]

Isn't this already done automatically in our rollup configuration at : https://github.com/mapbox/mapbox-gl-js/blob/42f00ac3e963f6dda9f4ed5390201e5ff25fa41a/build/rollup_plugins.js#L20

[mourner]

@asheemmamoowala what this does is tell Buble to transpile object spreads to Object.assign calls. But this will break IE11 because it doesn't have Object.assign. To make this work, we need to include a polyfill in the bundle.

[karimnaaji]

Addressed as part of the IE11 deprecation in v2.

[ryanhamley]

The issue here is no longer IE11 since GL JS no longer supports it, but some build systems fail when encountering the Object spread notation. See https://github.com/mapbox/mapbox-gl-js/issues/11266 for an example. We probably don't need to polyfill Object.assign either.