Remove additional exports of breakpoint symbols.

Restore typedef entries in `Breakpoint` component.
Generate type definitions for `constants.js` and `breakpointObserver.js`.
Validate size passed to `smallerThan`/`largerThan`.
Document `breakpointObserver`/`breakpoints`.
Fix `Breakpoint` component docs/example.

src/Breakpoint/breakpointObserver.d.ts

@@ -0,0 +1,19 @@
+/**
+ * Creates a readable store that returns the current {@link BreakpointSize}.
+ * It also provides functions for creating derived stores used to do comparisons.
+ */
+export function breakpointObserver(): {
+    subscribe: (this: void, run: import("svelte/store").Subscriber<any>, invalidate?: (value?: any) => void) => import("svelte/store").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) => import("svelte/store").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) => import("svelte/store").Readable<boolean>;
+};