Make completions an opt-in feature

[charliermarsh]

Summary

We now expect an initializationOptions setting to enable completions.

Closes #7.

Test Plan

• Hardcoded TEST as the returned completions in the LSP.
• Ran cargo build -p ty in the Ruff repo.
• Set the hardcoded path in the VS Code extension:

{
    "ty.path": [
        "/Users/crmarsh/workspace/ruff/target/debug/ty"
    ]
}

• Started the extension.
• Verified that no completions appeared.
• Enabled the setting:

{
    "ty.path": [
        "/Users/crmarsh/workspace/ruff/target/debug/ty"
    ],
    "ty.experimental.completions.enable": true
}

• Verified that the TEST completion appeared.

[charliermarsh]

Note that if you have a Python LSP enabled at all, you won't see these. Even if you uninstall Pylance, by default VS Code will fall back to Jedi, so you have to set "python.languageServer": "None" for this to matter.

[BurntSushi]

Oh interesting. I had thought the intent was to do this on the server side. Because this only impacts the VS Code extension right? Not anything else. (Maybe it's only worth doing for the VS Code extension.)

[charliermarsh]

Both are required. There's a change in ruff to respect this on the server: astral-sh/ruff#17921. Any client can pass in the necessary initialization options.