Declare general workout AST

src/ast.ts

@@ -0,0 +1,13 @@
+export type Workout = {
+  name: string;
+  author: string;
+  description: string;
+  intervals: Interval[];
+};
+
+export type Interval = {
+  type: "Warmup" | "Cooldown" | "Interval" | "Rest";
+  duration: number;
+  power: { from: number; to: number };
+  cadence?: number;
+};