Add CustomEvent polyfill from Mozilla docs

[bkjohnson]

Description

This PR adds a polyfill for CustomEvent since it isn't supported natively in IE11. This API is necessary for the analytics coming from component-library components.

The polyfill was copy-pasted with light modifications from https://developer.mozilla.org/en-US/docs/Web/API/CustomEvent/CustomEvent#polyfill
We are using core-js for several other polyfills, but that package has chosen not to support CustomEvent.

Relevant slack thread.

Testing done

1. @U-DON narrowed the problem in vets-website down to a dependency upgrade for component-library from 2.3.1 to 2.4.0. changes
2. Confirmed that the problem was with the dispatchAnalyticsEvent call by commenting it out and putting a simple console.log('hi') in the lifecycle method - the hca app loaded without the Object does not support this action error and we saw the expected log message.
3. Uncomment the event code and rebuild the dependency.
4. Add the polyfill in vets-website
5. Try to load an app and verify that it loads as expected
6. Put another console.log in the analytics event handler to verify that the CustomEvent is being fired.

Screenshots

Acceptance criteria

• [ ]

Definition of done

• Events are logged appropriately
• Documentation has been updated, if applicable
• A link has been provided to the originating GitHub issue (or connected to it via ZenHub)
• No sensitive information (i.e. PII/credentials/internal URLs/etc.) is captured in logging, hardcoded, or specs

[va-vfs-bot]

ESLint is disabled

vets-website uses ESLint to help enforce code quality. In most situations we would like ESLint to remain enabled.

What you can do

See if the code can be refactored to avoid disabling ESLint, or wait for a VSP review.

[bsmartin-ep]

LGTM