Improve istyle type

[hedwiggggg]

Description

Refactor IStyle typings for better typescript support.

Example

See playgrounds at #850

Packages

List all packages that have been changed.

• fela
• fela-plugin-fallback-value
• fela-plugin-typescript

Versioning

Patch

Checklist

Quality Assurance

You can also run yarn run check to run all 4 commands at once.

• The code was formatted using Prettier (yarn run format)
• The code has no linting errors (yarn run lint)
• All tests are passing (yarn run test)
• There are no flow-type errors (yarn run flow)

Changes

If one of the following checks doesn't make sense due to the type of PR, just check them.

• Tests have been added/updated
• Documentation has been added/updated
• My changes have proper flow-types

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