fix(core): bind this context to browser-based btoa and atob utilities

[dan-lovelace]

Description of changes

Fixes a Firefox-specific issue where atob() fails when called without proper Window context binding.

I received a vague error when trying to invoke the utility method decodeJWT inside Firefox: Error: Invalid token payload. After debugging, I noticed the error was coming from the convert method:

TypeError: 'atob' called on an object that does not implement interface Window.

Issue #, if available

N/A

Description of how you validated changes

I ran yarn build, then used the link instructions in the contributing guide to override my project's @aws-amplify/core dependency to my local, and tested the changes in both Firefox and Chrome. After the changes, the error is no longer thrown in Firefox. decodeJWT now works in Chrome and Firefox without error.

Checklist

• PR description included
• yarn test passes
• Unit Tests are changed or added
• Relevant documentation is changed or added (and PR referenced)

Checklist for repo maintainers

• Verify E2E tests for existing workflows are working as expected or add E2E tests for newly added workflows
• New source file paths included in this PR have been added to CODEOWNERS, if appropriate

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.