feat(CalendarEvent): Add support for status, i.e. free vs busy (#113)

* Introduce an enumerator to represent calendar status. Afaik, only two
  exist, but should make the code more readable as the notion of status
will be used in multiple places (extractors, ics.js, options page, ...)
* Add the atribute to the class CalendarEvent

src/js/utils/event.js

@@ -1,3 +1,45 @@
+/**
+ * Enumerator-like class to represent calendar event status. When an
+ * {@link CalendarEvent} is added to calendar clients, it may show as busy or
+ * free time slots.
+ *
+ * When you add an exam, you want it to show as "Busy" so that you do not
+ * schedule other apointements that colide with the exam event. On the other
+ * hand, events that are more like reminders, such as bill and library
+ * deadlines, you might prefer to set the event as "Free" as it does not
+ * consume time in your agenda.
+ *
+ * The implementation is inspired in
+ * {@link https://2ality.com/2020/01/enum-pattern.html}. Could be more robust,
+ * but I think is "enum"-ish enough :)
+ */
+class CalendarEventStatus {
+    /** @private */
+    static _allowed = ["Free", "Busy"];
+
+    static FREE = new CalendarEventStatus("Free");
+    static BUSY = new CalendarEventStatus("Busy");
+
+    constructor(name) {
+        if (!CalendarEventStatus._allowed.includes(name))
+            throw Error(`Unknown value '${name}'. Supported values are: ${CalendarEventStatus._allowed.join(",")}`);
+        this.name = name;
+    }
+
+    static fromValue(val) {
+        if (val == "Free") return CalendarEventStatus.FREE;
+        else if (val == "Busy") return CalendarEventStatus.BUSY;
+    }
+
+    value() {
+        return this.name;
+    }
+
+    toString() {
+        return `CalendarEventStatus.${this.name}`;
+    }
+}
+
 /**
  * Represents a calendar event instance, e.g. an exam, a recurring class or
  * a bill deadline
@@ -42,6 +84,8 @@ class CalendarEvent {
         this.recurStart = null;
         /** @type {Date?} */
         this.recurEnd = null;
+        /** @type {CalendarEventStatus} */
+        this.status = CalendarEventStatus.BUSY;
     }
 
     /**