Interactive Code Block: Extensibility

[adamziel]

Description

Let's enable extending the interactive code block with predefined:

• Libraries
• Execution scripts
• Output formatters

I put a "SQLite Battery" in a separate repo to illustrate what I mean here.

Rationale

I had to ship a custom output formatter called Tabular SQL queries in WordPress/wordpress-playground#168 to support the following post about SQLite integration for WordPress:

I expect to need more of these and I'd rather not pollute the core repository with one-off utilities for very specific posts.

Done is

I can add a targeted set of libraries, execution scripts, and output formatters via wp-admin.