add ForestType.Expensive enum for debugging

[daesunp]

Description

This PR adds another enum option ForestType.Expensive, which is passed in to set a flag in ObjectForest used to have more expensive asserts/validation for debugging.

[msfluid-bot]

⯅ @fluid-example/bundle-size-tests: +499 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	391.55 KB	⯅ +141 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	382 KB	⯅ +134 Bytes
Total Size	3.26 MB	3.26 MB	⯅ +499 Bytes

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Baseline commit: 2cb1e358aa231429b72c7fb3fce95e4b6e66c8a9

Generated by :no_entry_sign: dangerJS against ad77c67cccb9202e986e2d4c82e36a80ffc8a77e