Add a TreeAlpha.key2 method

[daesunp]

Description

This PR adds a TreeAlpha.key2 method which is meant to eventually replace our public Tree.key method. This new method now returns undefined in the case where there is a root node.

[noencke]

nit

Suggested change

	const propertyKey = getPropertyKeyFromStoredKey(parentSchema, storedKey);
	return getPropertyKeyFromStoredKey(parentSchema, storedKey);

[daesunp]

updated with suggested change

[Josmithr]

Left a couple nitpicks, but otherwise looks good!

[github-actions]

🔗 No broken links found! ✅

Your attention to detail is admirable.

linkcheck output

> fluid-framework-docs-site@0.0.0 ci:check-links /home/runner/work/FluidFramework/FluidFramework/docs
> start-server-and-test "npm run serve -- --no-open" 3000 check-links

1: starting server using command "npm run serve -- --no-open"
and when url "[ 'http://127.0.0.1:3000' ]" is responding with HTTP status code 200
running tests using command "npm run check-links"


> fluid-framework-docs-site@0.0.0 serve
> docusaurus serve --no-open

[SUCCESS] Serving "build" directory at: http://localhost:3000/

> fluid-framework-docs-site@0.0.0 check-links
> linkcheck http://localhost:3000 --skip-file skipped-urls.txt

Crawling...

Stats:
  210260 links
    1663 destination URLs
    1895 URLs ignored
       0 warnings
       0 errors