Add more license symbol functionality

[Rylan12]

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Modify some license functionality to allow for more license symbols to be added as needed. Specifically, this adds the :cannot_represent symbol as discussed in Homebrew/homebrew-core#63226 (comment).

In the future, adding the symbol to the ALLOWED_LICENSE_SYMBOLS allow list should be the only change necessary to allow for more license symbols.

[MikeMcQuaid]

Looks good, nice work again @Rylan12!