Following changes introduced in this PR:
* jsWasmMain and jsWasmTest sourcesets renamed to webMain and webTest
(it causes less confusion, regarding the fact that we already have
wasmJsMain, apart from that, this would be a standard name for this
sourceset in webMain)
* remove redunant resource processing (there's yet room for improvement
though)
   * Simplify wasm task introduction
* Remove flag for building js separately from wasm (this flag was a lie
- we can not have separate js target)