Emit events from Unhydrated SharedTree Nodes

[noencke]

Description

Currently, unhydrated nodes can be edited but they do not emit any change events. This PR fixes that by properly emitting events. Changes include:

• Removing the anchor parameter from the relevant events on AnchorEvents. This allows those events to be implemented by things that don't have anchors (like UnhydratedFlexTreeNode) and things that only sometimes have anchors (like TreeNodeKernel). Nobody uses that parameter currently anyway because it is redundant (it's the same as the object that the event is being registered on).
• Emitting a change event from UnhydratedFlexTreeNode when one of its fields is edited. This is listened to by the TreeNodeKernel, which can then emit its own corresponding event.
• Adding distinct types for the "unhydrated" and "hydrated" versions of the state in TreeNodeKernel. This makes it more obvious what the state transition is, and makes the type checking and safety more explicit.
• Adding a lazy function helper to better encapsulate the lazy-getting of the events in TreeNodeKernel.

[msfluid-bot]

⯅ @fluid-example/bundle-size-tests: +1.18 KB

Metric Name	Baseline Size	Compare Size	Size Diff
aqueduct.js	461.07 KB	461.11 KB	⯅ +35 Bytes
azureClient.js	558.69 KB	558.74 KB	⯅ +49 Bytes
connectionState.js	680 Bytes	680 Bytes	■ No change
containerRuntime.js	261.33 KB	261.35 KB	⯅ +14 Bytes
fluidFramework.js	401.93 KB	402.42 KB	⯅ +496 Bytes
loader.js	134.17 KB	134.19 KB	⯅ +14 Bytes
map.js	42.43 KB	42.44 KB	⯅ +7 Bytes
matrix.js	146.8 KB	146.8 KB	⯅ +7 Bytes
odspClient.js	525.84 KB	525.89 KB	⯅ +49 Bytes
odspDriver.js	97.8 KB	97.82 KB	⯅ +21 Bytes
odspPrefetchSnapshot.js	42.76 KB	42.78 KB	⯅ +14 Bytes
sharedString.js	163.79 KB	163.8 KB	⯅ +7 Bytes
sharedTree.js	392.4 KB	392.88 KB	⯅ +489 Bytes
Total Size	3.3 MB	3.3 MB	⯅ +1.18 KB

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Baseline commit: a9a07c6

Generated by 🚫 dangerJS against 9d48431