Add more build step types

[lylythechosenone]

Currently there is only one: Clone. Building an app with this singular instruction is quite difficult, so I think it's safe to say we need more step types.

Some examples might include:

• Copy - copy files from one location to another
• Command - run a shell command/shell script

Additionally, packages should be able to specify their own build steps. For example, when included as a build dependency, rust could have a cargo build step that runs cargo build in a given directory.