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/load/tests/platform.roc

@@ -0,0 +1,8 @@
+platform "test-platform"
+    requires {} { main : * }
+    exposes []
+    packages {}
+    provides [mainForHost]
+
+mainForHost : {} -> {}
+mainForHost = \{} -> {}