Update event tracking to more closely align with gtag

## Feature Description

Site Kit has long had a dedicated utility for internal event tracking instead of using `gtag` directly. This simplifies many things such as conditional calling based on whether or not the user has opted-in as well as the application of common parameters.

Since the implementation was initially built on UA, things like `event_category` were also built into the function as positional arguments.

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_Do not alter or remove anything below. The following sections will be managed by moderators only._

## Acceptance criteria

- `trackEvent` should be updated to accept an alternate signature which should be the preferred method going forward:
```ts
trackEvent( name: string, params?: Object )
```
- `name` above maps to the current `action` arg (i.e. event name)
- `params` would be merged with the others that make up the event data and should use the appropriate keys accordingly
- any `undefined` values should be excluded from the resulting data
- The current signature should be allowed for backwards compatibility and all other functionality should remain unchanged

## Implementation Brief

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### Test Coverage

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## QA Brief

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## Changelog entry

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