tree2: add sequence mutators

[taylorsw04]

This PR implements sequence mutation methods. Tests will be added in a follow-up PR when #17475 is complete.

[Josmithr]

What are the merge semantics here? I think it would be valuable to call them out (here and in other methods as appropriate). Are we inserting after the previous item? Before the following item?

[taylorsw04]

Neither; all methods shipped as part of MVP are index-centric, so they will result in insertion at the index specified unless concurrent changes push them forward (first-write-wins). When we introduce anchored changes, we can add those docs, but for now it's intuitive enough to omit (I validated this opinion with Noah/Alex).

[Josmithr]

Okay. That's probably fair. Still might be worth noting that. If I were looking at this API (as I was as a reviewer), that is one of the first questions I would be asking.

[Josmithr]

Is the intention to add unit tests for the new behaviors in this PR, or separately?

[Josmithr]

This doesn't seem user friendly IMO. Unless I'm missing something. The consumer is required to do the necessary offset math to figure out what index would be if the selected items were not in the list. Couldn't we do that math for them?

[noencke]

Yea, we are planning to do that at a lower level (i.e. inside of the sequence editor code). If that doesn't land by MVP, we can do the math at this higher layer, I suppose.

[msfluid-bot]

⯅ @fluid-example/bundle-size-tests: +956 Bytes

Metric Name	Baseline Size	Compare Size	Size Diff
aqueduct.js	443.96 KB	443.96 KB	■ No change
connectionState.js	680 Bytes	680 Bytes	■ No change
containerRuntime.js	237.29 KB	237.29 KB	■ No change
loader.js	148.39 KB	148.39 KB	■ No change
map.js	48.43 KB	48.43 KB	■ No change
matrix.js	142.21 KB	142.21 KB	■ No change
odspDriver.js	90.46 KB	90.46 KB	■ No change
odspPrefetchSnapshot.js	42.13 KB	42.13 KB	■ No change
sharedString.js	162.86 KB	162.86 KB	■ No change
sharedTree2.js	257.88 KB	258.81 KB	⯅ +956 Bytes
Total Size	1.68 MB	1.68 MB	⯅ +956 Bytes

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Baseline commit: 14d4581

Generated by 🚫 dangerJS against d6ba598

[DLehenbauer]

@taylorsw04 - I'm in the market for some sequence mutators to write test cases... hope you don't mind my merging. :-)