NO-TICKET: Expose 'informational' as part of the AlertVariation type

[Josh68]

Summary

When using Design System types, directly, in a project, the AlertVariation type is missing the string for the default Alert style, which is "informational," according to the docs. This update just adds that string to the union, but excludes it from the prop typing, since it's the default.

We are passing this detail via Adobe Analytics, so would like to have the "informational" typing exposed.

How to test

There is no impact to the use of the component. When built, a project that imports AlertVariation from Design System types should see "informational" as one of the options (in our case, for analytics purposes).

Checklist

• Prefixed the PR title with the Jira ticket number as [WNMGDS-####] Title or [NO-TICKET] if this is unticketed work.
• Selected appropriate Type (only one) label for this PR, if it is a breaking change, label should only be Type: Breaking
• Selected appropriate Impacts, multiple can be selected.
• Selected appropriate release milestone

If this is a change to design:

• If visual regression image references have been changed, design MUST be assigned to review. In this instance, designer approval is a requirement before the PR can be merged.

If this is a change to code:

• Created or updated unit tests to cover any new or modified code
• If necessary, updated unit-test snapshots (yarn test:unit:update) and browser-test snapshots (yarn test:browser:update)

If this is a change to documentation:

• Checked for spelling and grammatical errors