Add ThemeContext to TypeScript typedef

[Korzun]

Description

Adds ThemeProvider export to the react-fela TypeScript typedef; it already appears in index.js.

The index.d.ts's ThemeProviderProps and FelaThemeProps definitions both reference a Theme as having the type object, I maintained that choice.

Example

TypeScript Error

index.js

Packages

• react-fela

Versioning

Patch

Checklist

Quality Assurance

• 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

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

[Korzun]

Mentioned in #757 (comment)

[robinweser]

Thank you!