Flow: add support for 'implies' type guard variant

[gkz]

Description

Adds support for Flow's implies x is T type guards, along with the basic one and the asserts varient.

Checklist

• [x] I’ve added tests to confirm my change works.
• [x] (If the change is user-facing) I’ve added my changes to changelog_unreleased/*/XXXX.md file following changelog_unreleased/TEMPLATE.md.
• [x] I’ve read the contributing guidelines.

✨Try the playground for this PR✨