PartialVersion parsing support

Creating this issue to discuss about for PartialVersion parsing. Currently [`Version::parse`](https://docs.rs/semver/latest/semver/struct.Version.html#method.parse) only supports parsing full versions (i.e., with at least a major, minor, and a patch).

Would you be against adding support for PartialVersion parsing, e.g., in the same manner that [Cargo handles partial versions](https://github.com/rust-lang/cargo/blob/92f94a54ac8c25aff5f1051ebe59d8760539eac9/crates/cargo-util-schemas/src/core/partial_version.rs#L7)? This would involve adding a new struct PartialVersion, that supports parsing incomplete versions, and implementing `impl From for Version`

I'd be happy to submit a PR to add the struct & parsing if there's interest.