Track the Lightning CSS version in the gem version

[marcoroth]

This pull request makes the gem version mirror the Lightning CSS release it packages, so gem v1.33.0 bundles Lightning CSS v1.33.0, and the upstream workflow bumps that version along with the dependency.

Lightning CSS numbers its crate differently from its releases. Every version on crates.io is 1.0.0-alpha.N, with no stable release ever published, while the same build is tagged v1.33.0 and ships on npm as 1.33.0.

Following the crate would make every release a RubyGems prerelease, which bundle add lightningcss skips, so the gem follows the release instead:

LightningCSS::VERSION             #=> "1.33.0"
LightningCSS.lightningcss_version #=> "1.0.0-alpha.72"

The workflow reads the latest release from GitHub, then reads Cargo.toml at that tag to learn which crate the release carries:

v1.33.0  ->  1.0.0-alpha.72
v1.32.0  ->  1.0.0-alpha.71
v1.30.2  ->  1.0.0-alpha.68