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/test_syntax/tests/snapshots/fail/imports_missing_comma.header.result-ast

@@ -1 +1 @@
-Header(Imports(ListEnd(@87), @65))
+Header(Exposes(ListEnd(@11), @4))

crates/compiler/test_syntax/tests/snapshots/fail/imports_missing_comma.header.roc

@@ -1,4 +1 @@
-app "test-missing-comma"
-    packages { pf: "platform/main.roc" }
-    imports [pf.Task Base64]
-    provides [main, @Foo] to pf
+app [main, @Foo] { pf: platform "platform/main.roc" }