Add Decodal formatter

doc/manual/souce/formatting.md

@@ -0,0 +1,50 @@
+# Formatting
+
+Decodal includes an initial source formatter for parsed Decodal code.
+The formatter is available from the Rust crate, the CLI, the WebAssembly package, and the playground.
+
+## CLI
+
+Print formatted source to stdout:
+
+```sh
+decodal fmt config.dcdl
+```
+
+Format in place:
+
+```sh
+decodal fmt --write config.dcdl
+```
+
+Read from stdin:
+
+```sh
+cat config.dcdl | decodal fmt -
+```
+
+## Rust
+
+```rust
+let formatted = decodal::format_source("Server={port=Int default 8080;};")?;
+```
+
+## WebAssembly
+
+```js
+import init, { formatSource } from 'decodal-wasm';
+
+await init();
+const result = JSON.parse(formatSource('Server={port=Int default 8080;};'));
+
+if (result.ok) {
+  console.log(result.source);
+} else {
+  console.error(result.error);
+}
+```
+
+## Current limitation
+
+The formatter currently rejects source containing line comments rather than silently dropping them.
+Comment-preserving formatting requires attaching comments to syntax nodes or formatting from a concrete syntax tree, and should be implemented before comment-heavy source is formatted automatically.