Multiplatform progress bar rewrite

[ajalt]

This PR adds a new new implementation of all the progress layout and animation code with a lot of new features.

• Multiplatform support
• Coroutine support
• Multiple tasks that can be aligned or unaligned, without needing separate progress animations
• Better performance with less flickering
• New progress cells types
• Optional task context to change displayed text while the animation is running
• No internal classes; everything should be fully extensible now

The old APIs are left for backwards compatibility for now, although they're now wrappers around the new implementation.

Fixes #148
Fixes #141
Fixes #121
Fixes #152