Use default target hierarchy to setup build targets

[whyoleg]

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Fixes #147

Note: using applyDefaultHierarchyTemplate with extended hierarchy is experimental + accessors for WasmJs and WasmWasi will be available only in KGP 2.0.

Feel free to integrate it now or wait when above points will be resolved.

Minor additional changes:

• remove IR in JS target, as it's default now
• minor changes to use nicer APIs for js testTasks and sourceSet dependencies
• group native targets declaration by platform, to better understand which targets are enabled - maybe it's fine to even inline nativeTargets function, so that all targets will be in one place