Add proc-macro-server command (#1679)

**Stack**:
- #1686
- #1685
- #1684
- #1683
- #1718
- #1682
- #1681
- #1680
- #1679 ⬅
- #1715
- #1676


⚠️ *Part of a stack created by [spr](https://github.com/ejoffe/spr). Do
not merge manually using the UI - doing so may have unexpected results.*

scarb/src/bin/scarb/args.rs

@@ -178,6 +178,8 @@ pub enum Command {
         codes of selected packages. Resulting files will be placed in `target/package` directory.
     ")]
     Package(PackageArgs),
+    /// Start proc macro server.
+    ProcMacroServer,
     /// Upload a package to the registry.
     #[command(after_help = "\
         This command will create distributable, compressed `.tar.zst` archive containing source \

scarb/src/bin/scarb/commands/mod.rs

@@ -22,6 +22,7 @@ pub mod manifest_path;
 pub mod metadata;
 pub mod new;
 pub mod package;
+mod proc_macro_server;
 pub mod publish;
 pub mod remove;
 pub mod run;
@@ -49,6 +50,7 @@ pub fn run(command: Command, config: &mut Config) -> Result<()> {
         Metadata(args) => metadata::run(args, config),
         New(args) => new::run(args, config),
         Package(args) => package::run(args, config),
+        ProcMacroServer => proc_macro_server::run(config),
         Publish(args) => publish::run(args, config),
         Remove(args) => remove::run(args, config),
         Run(args) => run::run(args, config),

scarb/src/bin/scarb/commands/proc_macro_server.rs

@@ -0,0 +1,7 @@
+use anyhow::Result;
+use scarb::core::Config;
+
+#[tracing::instrument(skip_all, level = "info")]
+pub fn run(_config: &Config) -> Result<()> {
+    unimplemented!()
+}