tree2: Better and more consistent field cursor support

[CraigMacomber]

Description

This better formalizes the convention of using above root placeholder nodes, leverages this to provide field cursor builders, and makes cursor related function naming more consistent.

Breaking Changes

Some cursor related functions have been renamed and had argument order changed, but nothing was removed.

Reviewer Guidance

The review process is outlined on this wiki page.

[msfluid-bot]

⯅ @fluid-example/bundle-size-tests: +8 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	263.51 KB	263.51 KB	⯅ +8 Bytes
Total Size	1.72 MB	1.72 MB	⯅ +8 Bytes

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Baseline commit: 4892249

Generated by 🚫 dangerJS against 6063030

[Josmithr]

Left a couple of documentation nitpicks. Otherwise looks good to me!