generate: add entry definition with sample function

[zippy]

As an hcdev user, I want the generate zome command to create a scaffold in the zome.json file of an entry type with a sample exposed function definition, and also code for that exposed function.

Currently the generate function just creates a bare-bones zome with a "main()" code function that doesn't really make sense, it needs to be fleshed out a bit with a sample entry_types definition.

[Connoropolous]

For this, take a look at the new examples of the scaffold for assemblyscript zomes...

Create a little folder with scaffold files, and then put a file in, and the write the code like you want app developers to see it when they generate one...
Use that include_str! macro