breaking(breakpoint): add breakpointObserver store (#1092)

* breaking: re-name event "on:match" to "on:change" in `Breakpoint.svelte`

* feat: add `breakpointObserver` read-only store

* refactor: export breakpoint constants from `breakpoints.js`

src/Breakpoint/breakpointObserver.d.ts

@@ -0,0 +1,22 @@
+import type { Readable, Subscriber, Unsubscriber } from "svelte/store";
+import type { BreakpointSize, BreakpointValue } from "./breakpoints";
+
+/**
+ * Creates a readable store that returns the current breakpoint size.
+ * It also provides functions for creating derived stores used to do comparisons.
+ */
+export function breakpointObserver(): {
+    subscribe: (this: void, run: Subscriber<any>, invalidate?: (value?: any) => void) => Unsubscriber;
+    /**
+     * Returns a store readable store that returns whether the current
+     * breakpoint is smaller than {@link size}.
+     * @param {BreakpointSize} size Size to compare against.
+     */
+    smallerThan: (size: BreakpointSize) => Readable<boolean>;
+    /**
+     * Returns a store readable store that returns whether the current
+     * breakpoint is larger than {@link size}.
+     * @param {BreakpointSize} size Size to compare against.
+     */
+    largerThan: (size: BreakpointSize) => Readable<boolean>;
+};