Prepare for ESM

[kanongil]

Support plan

• is this issue currently blocking your project? (yes/no): no
• is this issue affecting a production system? (yes/no): no

Context

• node version: n/a
• module version: 4.1.3
• environment (e.g. node, browser, native): node
• used with (e.g. hapi application, another framework, standalone, ...): hapi / standalone
• any other relevant information:

What problem are you trying to solve?

Remove default export (part of hapijs/hapi#4279).

Do you have a new or modified API suggestion to solve the problem?

Move Podium class to Podium export and make validate & decorate regular exports.