Arbitrarily sized ints

The current `Int` is an int64. We should support ints that are arbitrarily sized.

One open question here is: should Pkl's numbers be arbitrarily sized? Or should we introduce a new type, e.g. `BigInt`?

Start by reviewing the current Int representation and the language's numeric-type design. Resolve whether Pkl numbers should become arbitrarily sized or whether a separate BigInt type is needed. Done means the design decision is made and arbitrary-sized integer support is specified accordingly.

Written by the indexing model from the issue text.