Migrate language server startup to JSON configuration

[purefunctor]

Problem

Compiler PR purefunctor/purescript-iris#496 implements purefunctor's chosen direction for purefunctor/purescript-iris#487: replace --source-command and --diagnostics-on-open/save/change with mutually exclusive --config <JSON> and --config-file <PATH> startup options.

The extension currently constructs ['--source-command', resolvedConfig.sourceCommand] in src/extension.ts. That invocation will be rejected by the updated compiler when a custom source command is configured.

src/configuration.ts and package.json currently expose alexandrite.sourceCommand and the deprecated purescriptAnalyzer.sourceCommand alias. The extension has no diagnostic-trigger settings and relies on compiler defaults.

Compiler contract

The JSON contract already lives in the compiler's shared configuration crate and schema:
https://github.com/purefunctor/purescript-alexandrite/blob/main/compiler-lsp/configuration/configuration.schema.json

Defaults:

{
  "sources": { "kind": "spago" },
  "diagnostics": { "onOpen": true, "onSave": true, "onChange": false }
}

Custom discovery:

{
  "sources": {
    "kind": "command",
    "program": "spago",
    "arguments": ["sources"]
  }
}

• Missing or null fields retain defaults; {} and top-level null select defaults.
• Commands supply a nonempty executable and an optional array of individual arguments, without shell parsing.
• Configuration is read once before startup. Relative --config-file paths resolve from the server process working directory.
• Unknown fields (including an embedded $schema property), malformed settings, and file-read failures produce stderr errors and exit status 2.
• This change does not add configuration-file watching or runtime LSP configuration updates; applying changed startup settings still requires a restart.

Requested follow-up

• Update server argument construction to use the new JSON CLI transport, passing an inline JSON document as a single argument or a configuration file path. Do not emit removed flags for updated compiler versions.
• Choose and document the extension settings interface, including migration of existing sourceCommand settings and the deprecated alias. Represent executable/argv separately rather than guessing shell quoting by splitting an old command string.
• Preserve default Spago discovery and diagnostic behavior when settings are absent. Use the shared schema for any exposed diagnostic settings.
• Define compatibility behavior for older compiler versions explicitly, including actionable migration/error messages.
• Add tests for generated server arguments, default settings, the selected migration behavior, arguments containing spaces/empty strings, and configuration errors. Update extension settings descriptions and usage documentation.

Compiler implementation thread: https://ampcode.com/threads/T-01a089ad-03d0-70ff-8777-7b90149d0a66