add a "Demo App" #252

Job	Run time
Slow tests: typescript with pnpm	51s
Tests (test-app-location)	23s
Tests (addon-location)	14s
Tests (declarations-configuration)	31s
Lint	14s
Tests (monorepo with npm)	12s
Tests (monorepo with pnpm)	17s
Tests (rollup-build)	15s
Slow tests: defaults with pnpm	52s
Tests (addon-only)	16s
Slow tests: typescript with npm	1m 21s
Slow tests: defaults with npm	1m 41s
Slow tests: try-scenarios	2m 8s
	9m 15s

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