feat(cli): add 'cdk lsp' command to start the CDK Language Server over stdio

[megha-narayanan]

Editors and AI agents start a language server by running a command over stdio,
but the CDK Language Server (in @aws-cdk/cdk-explorer) has no such entry
point. This adds one, so any LSP-capable editor or agent can get CDK
diagnostics and CodeLens by pointing at cdk lsp.

Wires up cdk lsp as a CLI subcommand that starts the server on stdin/stdout
and runs until the client closes the channel.

• Adds @aws-cdk/cdk-explorer as a runtime dependency so the server ships in the CLI bundle, version-aligned with the user's CDK toolchain.
• Writes nothing to stdout (CLI logs already go to stderr), keeping the JSON-RPC channel clean. applicationDir comes from the LSP initialize options, so the command takes no arguments.
• Mirrors the cdk explore wiring from feat: add web server scaffold and cdk explore command (#1578) #1598

Fixes #

Checklist

• This change contains a major version upgrade for a dependency and I confirm all breaking changes are addressed
  • Release notes for the new version:

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