The wasm codebase assumed that clients want to represent text as a
string of characters. This is faster, but in order to enable backwards
compatibility we add a `TextRepresentation` argument to
`automerge_wasm::Automerge::new` to allow clients to choose between a
`string` or `Array<any>` representation. The `automerge_wasm::Observer`
will consult this setting to determine what kind of diffs to generate.
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