tree2: Remove FieldNode unboxing

[CraigMacomber]

Description

FieldNodes in editable-tree will no longer implicitly unbox.

This means unboxing now removes at most 2 tree levels (leaf in an unboxing field) instead of potentially infinite.
This simplifies typing a bit, fixes an issue where recursive field nodes were lossy when traversing enumerable own properties, and better aligns with how the Proxy API handles Lists.

Breaking Changes

Use .content to access the content of field nodes in cases where they no longer implicitly unbox to that.

Reviewer Guidance

The review process is outlined on this wiki page.

[msfluid-bot]

⯆ @fluid-example/bundle-size-tests: -124 Bytes

Metric Name	Baseline Size	Compare Size	Size Diff
aqueduct.js	447.51 KB	447.51 KB	■ No change
connectionState.js	680 Bytes	680 Bytes	■ No change
containerRuntime.js	240.61 KB	240.61 KB	■ No change
loader.js	178.94 KB	178.94 KB	■ No change
map.js	48.06 KB	48.06 KB	■ No change
matrix.js	141.84 KB	141.84 KB	■ No change
odspDriver.js	90.27 KB	90.27 KB	■ No change
odspPrefetchSnapshot.js	41.9 KB	41.9 KB	■ No change
sharedString.js	162.75 KB	162.75 KB	■ No change
sharedTree2.js	275.09 KB	274.97 KB	⯆ -124 Bytes
Total Size	1.73 MB	1.73 MB	⯆ -124 Bytes

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Baseline commit: de08611

Generated by 🚫 dangerJS against 06c0024