Update the version CLI argument parser to accept "p2"

PR #164 modified WASI-Virt to work with WASI version "p2" (that is: any WASI version in the `0.2.x` series). The CLI argument processing will need to be updated accordingly.

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> I think we should update [src/bin/wasi-virt.rs](https://github.com/bytecodealliance/WASI-Virt/blob/main/src/bin/wasi-virt.rs#L228-L233) to accept the `p#` formatted instead of `0.2.12`. Otherwise it's a bit confusing misleading since you really only have 1 choice.

_Originally posted by @ryan-surname-p in https://github.com/bytecodealliance/WASI-Virt/issues/164#issuecomment-5118228046_

Start in src/bin/wasi-virt.rs at the CLI argument-processing code around lines 228-233 and inspect how the WASI version argument is parsed. Update the accepted format so p2 is recognized for the 0.2.x WASI series, then verify the CLI accepts p2 and still handles its documented version input correctly.

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