add widevine installation

[yellowsink]

This PR adds Widevine installation to Chromium environments in Gluon.

It will install it if needed for any Chromium browser window.

If opted out with {widevine: false}, it will not.

It also exports await Gluon.ensureWidevine({forceBrowser, forceEngine}), to explicitly install it.

Tested platforms:

• [x] Linux x64
• [ ] Linux x86
• [ ] macOS x64
• [ ] macOS arm64
• [ ] Windows x64
• [ ] Windows x86
• [ ] Windows arm64

Also note: if the default should be changed to false, index.js:190:86

[CanadaHonk]

After discussion, it was agreed this would be better served as a separate "plugin"/dependency rather than in the core of Gluon.