[vscode] Namespace Iris client and server settings

[purefunctor]

Summary

Separate extension-owned launch configuration from editor-independent language-server configuration:

• move executable selection to iris.client.serverPath
• contribute source discovery and diagnostic triggers under iris.server.*
• launch Iris through its required lsp subcommand
• retain the flat iris.serverPath and iris.sourceCommand settings as deprecated migration aliases
• remove purescriptAnalyzer.* settings and automatic purescript-analyzer executable detection

This aligns the extension with the runtime workspace-configuration contract introduced by purefunctor/purescript-iris#502. Server settings are resource-scoped and can therefore be returned for the resource in Iris's workspace/configuration request.

Verification

Built the Iris server from the PR #502 checkout and ran the complete extension test suite against /home/user/workspace/repo/target/debug/iris:

• 15 unit tests passed
• 11 VS Code integration tests passed
• TypeScript checks and the production bundle passed

The integration workspace selects that binary through iris.client.serverPath and discovers fixture sources through iris.server.sources.