Add prepl-server task

[daveyarwood]

This adds a prepl-server built-in task, which is a thin wrapper around the existing socket-server task.

Motivation and Context

prepl-based tooling is starting to become a thing, and it's useful to be able to start a prepl server in the context of a Boot project with minimal effort.

How Has This Been Tested?

I have this task in my profile.boot and it works nicely for my personal workflow using Conjure (a prepl-based Clojure development plugin for Neovim).

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
  • I made an attempt... I ran ./mkdocs and it updated a lot of things in the docs, but weirdly, it didn't add prepl-server to the built in tasks docs, so I didn't check the doc updates in.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
  • Unclear to me whether it's needed in this case.
• All new and existing tests passed.