chore: hello-gno app example

[iuricmp]

Hello world gno example

This PR adds a new sample app under "examples/hello". This app is a minimal app that connects to the blockchain.

[jefft0]

I enter:

git clone https://github.com/iuricmp/gnomobile --branch hello
cd gnomobile
make build

It prints the error:

cd /Users/jefft0/work/gno/gnomobile/examples/hello; /Applications/Xcode.app/Contents/Developer/usr/bin/make node_modules
make[1]: *** No rule to make target `node_modules'.  Stop.

[iuricmp]

Thanks @D4ryl00

[jefft0]

I did make new-app APP_NAME=testapp. This builds and runs on a fresh macOS virtual machine for Android and iOS.