[FX-4529] Add split button support to Alert

[mkrl]

FX-4529

Description

According to BASE, the action buttons in Alert should also support their split variants with dropdown menus.
Each button can either be normal or split, with only two variants available (one of each at the time only).

How to test

• Temploy
• See if the code and API looks good
• Look at the new examples for Alert on the temploy
• Tests should pass

Screenshots

Development checks

• Add changeset according to guidelines (if needed)
• Read CONTRIBUTING.md and Component API principles
• Make sure that additions and changes on the design follow Toptal's BASE design, and it's been already discussed with designers at #-base-core
• Annotate all props in component with documentation
• Create examples for component
• Ensure that deployed demo has expected results and good examples
• Ensure the changed/created components have not caused accessibility issues. How to use accessibility plugin in storybook.
• Self reviewed
• Covered with tests (visual tests included)

All development checks should be done and set checked to pass the
GitHub Bot: TODOLess action

PR commands

List of available commands:

• @toptal-bot run package:alpha-release - Release alpha version
• @toptal-anvil ping reviewers - Ping FX team for review

PR Review Guidelines

When to approve? ✅

You are OK with merging this PR and

1. You have no extra requests.
2. You have optional requests.
   1. Add nit: to your comment. (ex. nit: I'd rename this variable from makeCircle to getCircle)

When to request changes? ❌

You are not OK with merging this PR because

1. Something is broken after the changes.
2. Acceptance criteria is not reached.
3. Code is dirty.

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2. You have an opinionated comment regarding the code that requires a discussion.
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How to handle the comments?

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2. An owner of a comment must resolve it when it's addressed.
3. A PR owner must reply with ✅ when a comment is addressed.

[changeset-bot]

🦋 Changeset detected

Latest commit: ac6bbe9

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@toptal/picasso	Minor

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Click here if you're a maintainer who wants to add another changeset to this PR

[mkrl]

@toptal-anvil ping reviewers

[sashuk]

Code looks good, tested on temploy!