fix: add cargo-binstall metadata for the prebuilt release assets

[MaximeGaudin]

Summary

• slint-cli ships prebuilt binaries in GitHub Releases but its Cargo.toml carried no [package.metadata.binstall] section, so cargo binstall had no way to find the assets and would fall back to compiling from source.
• Added binstall metadata with a per-target pkg-url override for each of the five published targets, pointing at the exact release asset name (slint-darwin-arm64.tar.gz, slint-linux-amd64.tar.gz, slint-windows-amd64.zip, ...). The templates use the { repo } and { version } keys and match the repo's tag scheme (tag equals the crate version, verified against the existing v0.2.0 release assets). pkg-fmt is tgz except zip for the Windows override.
• The per-artifact checksum half of this issue (.sha256 files published alongside the assets) is handled separately in the companion PR for release: no checksums published as downloadable release assets #201.

Verified: cargo metadata parses the manifest and exposes the binstall metadata.

Fixes #146

Test plan

• cargo metadata --no-deps parses the updated manifest (metadata present).
• Template URLs were checked by hand against the asset names and tag format of the existing v0.2.0 release.
• CI green on the PR