Add delete server option to Drasi Server VS Code extension

[Copilot]

The Drasi explorer let users add and edit server connections but provided no way to remove them. This adds a Delete server action to the explorer context menu and command palette.

Changes

• config.ts — New ConnectionRegistry.removeConnection(id). Drops the connection from the saved list; if it was the active one, promotes the first remaining connection (or clears currentConnectionId when none are left).
• drasi-explorer.ts — New deleteConnection handler registered as drasi.connection.delete. Resolves the target from the clicked node or, when invoked without one (command palette), via a quick-pick. Prompts for Yes/No confirmation before deleting, then refreshes the tree.
• package.json — Command definition plus view/item/context entry (shown on current and non-current server nodes in the 3_danger group) and a commandPalette entry.
• Docs — Added a "Delete a Server" section to the extension README and updated the server-management bullet in the root README.

Notes for reviewers

• Confirmation dialog and 3_danger menu group mirror the existing deleteResource flow for consistency.
• No unit test added: the extension's test suite is VS Code integration-only and ConnectionRegistry depends on live VS Code configuration storage, so there is no existing harness to cover it.