fix(functions): Set default functions region and add documentation - fix #1945

[paulstelzer]

Checklist

• Issue number for this PR: Support region configuration via FunctionsRegionToken is a BREAKING CHANGE #1945
• Docs included?: No
• Test units included?: No
• In a clean directory, yarn install, yarn test run successfully? Yes

Description

Added a FunctionsDefaultRegion, so the user does not need to add a FunctionsRegionToken to his ngModule.

I also edited the documentation to use the FunctionsRegionToken.

IMPORTANT: I added FunctionsDefaultRegion as constant directly int the functions.ts . If you manage those constants anywhere also or inside the class, please change it!

[jamesdaniels]

Thanks for the fix!