Add sliding dots typing indicator to Fluent theme

[compulim]

Changelog Entry

Added

• Added sliding dots typing indicator in Fluent theme, in PR #5447, by @compulim

Description

Adding sliding dots style typing indicator to Fluent theme.

Design

Currently, the test does not use prefers-reduced-motion to pause the animation because SMIL animation cannot be paused via CSS.

We will need to work on this.

Specific Changes

• Refactored the sliding dot animation into assets, from the load completion animation
• Modified typing indicator in Fluent/Copilot theme to use the sliding dot animation

• I have added tests and executed them locally
• I have updated CHANGELOG.md
• I have updated documentation

Review Checklist

This section is for contributors to review your work.

• Accessibility reviewed (tab order, content readability, alt text, color contrast)
• Browser and platform compatibilities reviewed
• CSS styles reviewed (minimal rules, no z-index)
• Documents reviewed (docs, samples, live demo)
• Internationalization reviewed (strings, unit formatting)
• package.json and package-lock.json reviewed
• Security reviewed (no data URIs, check for nonce leak)
• Tests reviewed (coverage, legitimacy)

[OEvgeny]

Bear in mind that svg is 2-3x less performant. (meaning svg in the same window context).