Prerelease extensions are also showing the `Preview` badge

[filiptronicek]

After the addition of prereleases in #410, the extensions that leverage it automatically have the Preview label shown on them, but that should not happen, because they already have the pre-release badge.

OpenVSX: (see both Pre-Release & Preview badges are here)

VS Code Marketplace (see only the Pre-Release badge is present)

cc @jeanp413