dev container 'features' vscode-specific properties should be nested under `customizations.vscode`

[joshspicer]

Per the "supporting tools" spec, and to reduce confusion, we should migrate the three vscode-specific properties to be nested under customizations

The features proposal spec should also be updated to include these properties at the same time as the reference implementation: https://github.com/devcontainers/spec/blob/main/proposals/devcontainer-features.md

[chrmarti]

The CLI has code updating existing features to use "customizations" (containerFeaturesConfiguration.ts#L609) for compatibility. The schema needs to be updated.

[jkeech]

The schema was updated in devcontainers/spec#65. Closing as completed, but please re-open if there's more to do here.