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`

docs/src/pages/framed/Breakpoint/Breakpoint.svelte

@@ -5,13 +5,13 @@
   let events = [];
 </script>
 
-<Breakpoint bind:size on:match="{(e) => (events = [...events, e.detail])}" />
+<Breakpoint bind:size on:change="{(e) => (events = [...events, e.detail])}" />
 
 <p>Resize the width of your browser.</p>
 <h6>Breakpoint size</h6>
 <h1>{size}</h1>
 
-<h6>on:match</h6>
+<h6>on:change</h6>
 <pre>
   {JSON.stringify(events, null, 2)}
 </pre>

docs/src/pages/framed/Breakpoint/BreakpointObserver.svelte

@@ -0,0 +1,15 @@
+<script>
+  import {
+    breakpointObserver,
+    breakpoints,
+  } from "carbon-components-svelte/src/Breakpoint";
+
+  const size = breakpointObserver();
+  const smaller = size.smallerThan("md");
+  const larger = size.largerThan("md");
+</script>
+
+<p>Current breakpoint size: {$size}</p>
+<p>Current breakpoint value: {breakpoints[$size]}px</p>
+<p>Smaller than medium: {$smaller}</p>
+<p>Larger than medium: {$larger}</p>