Update REPL to use ext.Strings formatter when possible.

[jnthntatum]

ext.Strings provides a string formatter that formats builtin values in a nice format that
generally works as a CEL fragment.

This addresses #656 for basic types, but doesn't yet support extension types / protobuf
messages.

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[TristonianJones]

This looks good, but I think you need to update the repl/BUILD.bazel