Draft: Issue #3458215: Initial work for the button component

Closes #3458215

core/modules/navigation/components/toolbar-button/toolbar-button.component.yml

+$schema: https://git.drupalcode.org/project/sdc/-/raw/1.x/src/metadata.schema.json
+name: Admin Toolbar Button
+description: This a single button for the admin toolbar with an optional icon.
+status: stable
+group: Navigation
+
+props:
+  type: object
+
+  properties:
+    html_tag:
+      title: HTML Tag
+      type: string
+      enum:
+        - button
+        - a
+      default: a
+    icon:
+      title: Icon
+      type: string
+    extra_classes:
+      type: array
+      items:
+        type: string
+    action:
+      type: boolean
+    has_safe_triangle:
+      title: Has safe triangle?
+      description: Use this to improve this menu's usability.
+      type: boolean
+      label:
+        type: object
+        properties:
+          text:
+            title: Button label
+            type: string
+          attrs:
+            title: Label classes
+            description: Extra classes for the label
+            type: Drupal\Core\Template\Attribute
+
+slots:
+  content:
+    title: Content
+    required: true
+    description: The content of the button.