Add build subcommand: DSL spec to v2-map JSON file

Reads the DSL file, parses to AST, loads all --atlas-dir paths into
a registry, runs the builder, and writes pretty-printed JSON. The
output filename's basename becomes the map id when the DSL has no
'id' directive. Parser and builder errors are surfaced as
'<file>:<line>: <message>'. Also reorders the duplicate-atlas warning
so the atlas_id appears before the word 'duplicate'.

src/cli.js

@@ -32,6 +32,7 @@ function run(argv, { out, err }) {
   const handlers = {
     encode: require('./commands/encode'),
     decode: require('./commands/decode'),
+    build: require('./commands/build'),
   };
   const handler = handlers[head];
   if (!handler) {