Publish the JSON schema for `devcontainer-feature.json`

[joshspicer]

Context in the #devcontainer-community Slack

Similar to the devcontainer.json schema, publish the schema for devcontainer-feature.json.

Supporting tools (eg: The Dev Containers VS Code extension) can then pick up this schema to provide intellisense.

Subtasks

• [ ] Publish in spec repo (https://github.com/devcontainers/spec/pull/152)
• [ ] Publish on containers.dev
• [ ] Embed into VS Code extension for file validation

[chrmarti]

Related: https://github.com/devcontainers/spec/issues/93 (pending issue for main schema)