Add base package

[OEvgeny]

Fixes #

Changelog Entry

(No changelog entry for the change)

Description

Add a stub botframework-webchat-base package. In case we need something extremely useful to be shared across our packages, it'll help.

Design

Specific Changes

-

• Added base package
• Added build infra

• [ ] 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)