Add SchemaLock extension

[lgogolin]

Adds the SchemaLock extension: schema-pinned validation, completion, and hover for Kubernetes-style YAML, backed by a language server.

• Repo: https://github.com/schemalock/zed
• ID: schemalock
• Version: 0.1.0
• License: Apache-2.0

Notes

• Registers the schemalock language server for YAML. In a project rooted at a schemalock.yaml, owned resources get completion/hover/diagnostics from the pinned schema; unowned YAML is transparently proxied to yaml-language-server.
• The extension does not bundle a binary — it downloads the pinned per-platform language-server build from a public CDN on first use and caches it (same pattern as other LSP-backed extensions that fetch their server).
• Tested locally as a dev extension (loads, downloads the server, serves diagnostics/completion/hover).

Checklist

• Extension repo has an accepted license (Apache-2.0)
• Added as a git submodule under extensions/schemalock
• Entry added to extensions.toml (alphabetically sorted)
• Tested locally as a dev extension