[ImportVerilog] add unpacked array concatenation

[chenbo-again]

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This PR is aimed to provide unpacked array concatenation.

// unpacked array concatenation
string s[] = { "hello", "sad", "world" };
// string concatenation
string = { "hello", "sad", "world" };

these two operation do not act the same way, and the result of concat is depends on the Lhs.

close #8173