Implement new C# index syntax

[RReverser]

• Changes index declarations to subclassing+namespacing as [Index.BTree] to better convey that you can't have multiple properties on the same index (e.g. when we add Hash, you can't do [Index(BTree = ..., Hash = ...)].
• Adds support for inline [Index.BTree] attributes on fields.
• Adds various diagnostics for invalid index declarations.

Marking as WIP because the proposal that specifies this syntax is in review for now - https://github.com/clockworklabs/SpacetimeDBPrivate/pull/1225 - and the syntax might still change.

Fixes #2048.

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