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)

[compulim]

Also mark this package as private: true so we don't accidentally publish it.

[compulim]

Let's make this base package internal and bundled into other packages.

For a cleaner approach, we should use tsup for CJS build and no longer need Babel.