Add token-based PEG frontend and CLI source parsing

crates/frontend/examples/basic.endor

@@ -0,0 +1,9 @@
+// Parsing produces an AST; it does not execute these functions.
+function echo(text: String): String {
+    let result = text
+    return result
+}
+
+function discountedPrice(price: Number, discount: Number): Number {
+    return price - discount
+}