examples: add minimal_lsp.rs and FIFO test script

[shashi1687]

Adds a single example, examples/minimal_lsp.rs, plus a tiny FIFO test script.
The sample now shows:

- completion textDocument/completion
- hover textDocument/hover
- goto-definition textDocument/definition
- full-doc formatting (rustfmt) textDocument/formatting
- dummy diagnostics textDocument/publishDiagnostics
- UTF-8 offsetEncoding in the initialize response

Quick test

one-shot, all nine packets

cargo build --example minimal_lsp
target/debug/examples/minimal_lsp < examples/nine_packets.txt

Also includes examples/manual_test.sh, a tiny FIFO script that sends the canonical 9 LSP packets so the server can be verified from two terminals.

Two-terminal test

mkfifo /tmp/lsp_pipe
terminal A
cat /tmp/lsp_pipe | cargo run --example minimal_lsp
terminal B
bash examples/manual_test.sh # pumps the same nine packets

We should see, in order: capabilities → diagnostics → completion → hover → definition → formatting → shutdown.
(anyhow stays under [dev-dependencies]).

Fixes #20017

[shashi1687]

Hi @ShoyuVanilla @lnicola ,

Thank you for the helpful guidance on my last attempt on #20187
As you suggested, I’ve folded the logic from goto_def.rs and my earlier demo into a single file, examples/minimal_lsp.rs.
The new example now covers

• completion
• hover
• goto-definition
• full-file formatting via rustfmt
• a dummy diagnostic push
• UTF-8 offsetEncoding in the initialize

I’ve also added a tiny FIFO helper (examples/manual_test.sh) to replay the nine canonical packets, so the whole round-trip can be tested in two terminals.
Whenever you have a moment, I’d really appreciate another look.
Thanks again for your time and feedback!

— Shashi