tree: Avoid any, document and test UnionToIntersection

[CraigMacomber]

Description

Tweak UnionToIntersection to avoid any.
Document how it works.
Add tests.

Reviewer Guidance

The review process is outlined on this wiki page.

[msfluid-bot]

⯅ @fluid-example/bundle-size-tests: +245 Bytes

Metric Name	Baseline Size	Compare Size	Size Diff
aqueduct.js	456.49 KB	456.53 KB	⯅ +35 Bytes
azureClient.js	554.23 KB	554.28 KB	⯅ +49 Bytes
connectionState.js	680 Bytes	680 Bytes	■ No change
containerRuntime.js	257.7 KB	257.72 KB	⯅ +14 Bytes
fluidFramework.js	391.41 KB	391.43 KB	⯅ +14 Bytes
loader.js	134.1 KB	134.11 KB	⯅ +14 Bytes
map.js	42.17 KB	42.17 KB	⯅ +7 Bytes
matrix.js	145.43 KB	145.44 KB	⯅ +7 Bytes
odspClient.js	522.23 KB	522.28 KB	⯅ +49 Bytes
odspDriver.js	97.17 KB	97.19 KB	⯅ +21 Bytes
odspPrefetchSnapshot.js	42.27 KB	42.29 KB	⯅ +14 Bytes
sharedString.js	162.51 KB	162.52 KB	⯅ +7 Bytes
sharedTree.js	381.87 KB	381.88 KB	⯅ +7 Bytes
Total Size	3.26 MB	3.26 MB	⯅ +245 Bytes

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Baseline commit: 2cb1e35

Generated by 🚫 dangerJS against 449d807