Export provider context for class components to use

[waltjones]

Description of the change

Rollbar-react uses a React Context to make the Rollbar object available to all components. Function components can use the useRollbar hook, and this is the easiest way to get the Rollbar object.

Class components cannot use hooks, and so cannot use the useRollbar hook. React provides other ways to get the context, but the package currently lacks the necessary exports.

This PR adds the exports that allow class components to get the Rollbar object from the React Context, and adds the related type definitions so this will also work in Typescript.

Usage examples have been added to the related issue: #17

Type of change

• New feature (non-breaking change that adds functionality)

Related issues

Fixes #17

Development

• Lint rules pass locally
• The code changed/added as part of this pull request has been covered with tests
• All tests related to the changed code pass in development

Code review

• This pull request has a descriptive title and information useful to a reviewer. There may be a screenshot or screencast attached
• "Ready for review" label attached to the PR and reviewers assigned
• Issue from task tracker has a link to this pull request
• Changes have been reviewed by at least one other engineer