Consider more flexible syntax for `symbols` clause in a module.

[toddjonker]

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          > This clause expects a list of strings....

I wonder if we should expect a list of text values instead, allowing existing symbol IDs to be used in defining a module. $0 would lead to an entry with unknown text the way that non-string entities do today.

Originally posted by @zslayton in https://github.com/amazon-ion/ion-docs/pull/205#discussion_r1177860537