Prepare VS Code extension for publication

[dfreeman]

This PR prepares the Code extension for publishing to the Visual Studio Marketplace.

• Fills out the README, including a handful of screenshots of the extension in action
• Narrows activation criteria to only kick in for workspaces with a .glintrc file
• Adds extension metadata to our package.json
• Instructs yarn not to hoist any of the extension's dependencies, as vsce wants them all to be locally available during packaging.

On that final point, in the future we should consider bundling the extension source into a single module with via webpack/esbuild/similar, which should slightly improve startup time and eliminate the need to package the dependencies themselves.