feat: Unpublish rollbar_{dart,flutter} and add vendoring instructions

[mgalgs]

Fixes #124

Description of the change

Since Rollbar will no longer be publishing package updates to pub.dev, it will be helpful for many users to "vendor" the Rollbar packages into their projects so that they can have full control over package versions, pull in fixes from the community, etc. For example, the rollbar_dart package on pub.dev currently has an old http dependency, even though this has already been updated on the main branch.

Add instructions to the README for vendoring rollbar_flutter into a user project, and change the internal rollbar_{dart,flutter} dependencies to be "path" dependencies (pulling from within this repo) rather than "hosted" (pulling from pub.dev).

Type of change

• [ ] Bug fix (non-breaking change that fixes an issue)
• [ ] New feature (non-breaking change that adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [X] Maintenance
• [ ] New release

Related issues

• Fix #124
• Related: #131 #126

Checklists

Development

• [X] Lint rules pass locally
• [X] The code changed/added as part of this pull request has been covered with tests
• [X] 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