Make config.rs a single source of truth for configuration.

Configuration is editor-independent. For this reason, we pick
JSON-schema as the repr of the source of truth. We do specify it using
rust-macros and some quick&dirty hackery though.

The idea for syncing truth with package.json is to just do that
manually, but there's a test to check that they are actually synced.

There's CLI to print config's json schema:

    $ rust-analyzer --print-config-schema

We go with a CLI rather than LSP request/response to make it easier to
incorporate the thing into extension's static config. This is roughtly
how we put the thing in package.json.

crates/rust-analyzer/Cargo.toml

@@ -27,7 +27,7 @@ pico-args = "0.3.1"
 oorandom = "11.1.2"
 rustc-hash = "1.1.0"
 serde = { version = "1.0.106", features = ["derive"] }
-serde_json = "1.0.48"
+serde_json = { version = "1.0.48", features = ["preserve_order"] }
 threadpool = "1.7.1"
 rayon = "1.5"
 mimalloc = { version = "0.1.19", default-features = false, optional = true }

crates/rust-analyzer/src/bin/args.rs

@@ -28,6 +28,7 @@ pub(crate) enum Command {
     StructuredSearch { debug_snippet: Option<String>, patterns: Vec<SsrPattern> },
     ProcMacro,
     RunServer,
+    PrintConfigSchema,
     Version,
     Help,
 }
@@ -135,6 +136,10 @@ impl Args {
             return Ok(Args { verbosity, log_file: None, command: Command::Help });
         }
 
+        if matches.contains("--print-config-schema") {
+            return Ok(Args { verbosity, log_file, command: Command::PrintConfigSchema });
+        }
+
         let subcommand = match matches.subcommand()? {
             Some(it) => it,
             None => {

crates/rust-analyzer/src/bin/main.rs

@@ -31,6 +31,9 @@ fn try_main() -> Result<()> {
     setup_logging(args.log_file)?;
     match args.command {
         args::Command::RunServer => run_server()?,
+        args::Command::PrintConfigSchema => {
+            println!("{:#}", Config::json_schema());
+        }
         args::Command::ProcMacro => proc_macro_srv::cli::run()?,
 
         args::Command::Parse { no_dump } => cli::parse(no_dump)?,