Move the ES2015 compilation target setting to the `demo` project beca… #187

Job	Run time
check (ubuntu-latest)	7m 12s
dependency-submission (windows-latest)	7m 57s
dependency-submission (ubuntu-latest)	6m 23s
check (windows-latest)	7m 31s
dependency-submission (macos-latest)	5m 22s
check (macos-latest)	7m 36s
	42m 1s

Provide feedback