IE 11 polyfill for CSS variables

[JustSlone]

This is to add a polyfill for ie11 since it does not support css variables. Main work items are:

• Make a package to put this polyfill script and future ie11 polyfills
• Check in polyfill script to the package
• Test polyfill on component that uses css variables(Button in @fluentui/react-button)
• Write documentation for using the polyfill
• Repackage (soft fork)

For websites that use hot reloading, pages that need the polyfill script would need to be refreshed, or the polyfill script needs to be put in these pages.

[JustSlone]

@tomi-msft can you fill a brief description and any relevant information for this work?

lists like this are useful too

• Do thing
• do other thing

[msft-github-bot]

🎉This issue was addressed in #14567, which has now been successfully released as @uifabric/example-app-base@v7.15.4.:tada:

Handy links:

• Release Notes
• Official NPM Release

[msft-github-bot]

🎉This issue was addressed in #14567, which has now been successfully released as @fluentui/react-button@v0.11.5.:tada:

Handy links:

• Release Notes
• Official NPM Release

[msft-github-bot]

🎉This issue was addressed in #14567, which has now been successfully released as @fluentui/ie11-polyfills@v0.1.1.:tada:

Handy links:

• Release Notes
• Official NPM Release