Create AntiPatterns (working title) to marshall Patterns into something editor-friendly and back

[Pike]

When humans edit Fluent content, they're most often editing straight-forward patterns.

They shouldn't be bothered with leading/trailing whitespace, or escaping {.

Thus we should create a representation of Patterns which is editing-friendly.

Pontoon has code for similar purposes in https://github.com/mozilla/pontoon/tree/master/frontend/src/core/utils/fluent.

Working title is AntiPattern, as combining Patterns and AntiPatters is pure energy. This name should not stick.