Setup release CI

[sidvishnoi]

• To make sure we use right version in extension manifest.
• Be able to create zip, git tag and GitHub release in one go.
  • If we create a release commit, we can just update the version in package.json and use that version in manifest.
• Add a "preview" suffix to name/description for preview channel builds.
• I think we'll have preview channel builds by default. Only some of those builds will get promoted to release channel. So I think those GitHub releases will have additional .zip assets/artifacts (preview build for all releases preview-chrome-x.y.z.zip and a chrome-x.y.z.zip for promoted public releases)

Out of scope for now:

• auto-upload zip files to extension stores (in correct channel(s))