New app header syntax

Implements the new app header syntax as discussed in Zulip [1].

    app [main] {
	cli: platform "../platform/main.roc",
	json: "../json/main.roc"
    }

Old headers still parse and are automatically upgraded to the new
syntax by the formatter.

[1] 418444862

crates/compiler/parse/src/parser.rs

@@ -211,6 +211,8 @@ pub enum EPackageEntry<'a> {
     Shorthand(Position),
     Colon(Position),
     IndentPackage(Position),
+    IndentPlatform(Position),
+    Platform(Position),
     Space(BadInputError, Position),
 }