Add ability to extend template inside template.config.js file

[szymonrybczak]

Describe the Feature

The problem with maintaining template is that it needs to update with every new React Native version, whereas templates could implement their whole custom stuff inside postInitScript and maintainers could avoid extra work with upgrading template.

Template creator could just specify that the template extends default @react-native-community/template and CLI during init will instead of using template files from sourceDir property would copy the one that it extends.

The end results for app user would be the same, but the amount of work that needs to be done from the template maintainers is a lot smaller. It'd probably one time thing.

Possible Implementations

Inside template.config.js add extends property which accept template source (in the same format as --template option) and use it for init.