Extend `install` with additional tasks

[pepicrft]

When contributing to a project, developers usually go through a two-step process:

• Make sure the system dependencies are present. This is something they can do easily with rtx install.
• Run other project setup steps like installing NPM dependencies (this is usually documented in the README).

I think it'd be great if rtx could help integrate the latter into the rtx install, which users of the tool will get familiar with. I think install could support being extended with additional tasks. Each of those tasks should include the command/script that needs to run and a way to verify if the task needs to run (this can be a future improvement).