Add richer LSP server example (#20017)

[shashi1687]

What this PR does

This adds a new “head-start” LSP server example under:
crates/rust-analyzer/examples/rust_analyzer_lsp_example/
It demonstrates how to:

• Capture document edits with textDocument/didChange and log the URI
• Stub out textDocument/completion (returns an empty list)
• Stub out textDocument/formatting (returns no edits)

What the example does

1. Initialization
   • Opens JSON-RPC over stdio
   • Advertises full text sync, a completion provider, and a formatting provider
2. Main loop
   • Logs every buffer change (didChange)
   • Replies to completion requests with {"isIncomplete":false,"items":[]}
   • Replies to formatting requests with []
   • Handles clean shutdown on shutdown/exit

This gives anyone embedding rust-analyzer a minimal “plug-in” template for completions and formatting.

About the author’s note

“The current LSP example is a good, minimal demonstration. It shows how to respond to textDocument/definition. I used it to make an example that includes capturing content from textDocument/didChange and using it to provide textDocument/completion and textDocument/formatting functionality…”

• We’ve taken that idea and upstreamed it as a fresh, in-tree example.
• No external code was copied verbatim—just a clean re-implementation.

Testing & Validation

1. Build

   cargo build -p rust-analyzer --example rust_analyzer_lsp_example