ui: Add new component blueprint (#12421)

This adds a new component blueprint for all our components. We've been
using README.mdx files for quite some time to document our components
for other engineers. This adds a default file to help new engineers get
started writing useful documentation. These README.mdx file are also
very useful for building out components in isolation from scratch and
and some point will be used to run automated component testing.

ui/packages/consul-ui/blueprints/component/files/__root__/__templatepath__/README.mdx

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+<!-- START component-docs:@tagName -->
+# <%= classifiedModuleName %>
+<!-- END component-docs:@tagName -->
+
+```hbs preview-template
+<figure>
+
+  <figcaption>
+    Provide a widget for controlling things
+  </figcaption>
+
+  <select
+    onchange={{action (mut this.type) value="target.value"}}
+  >
+    <option>type-1</option>
+  </select>
+</figure>
+
+<figure>
+
+  <figcaption>
+    Provide a some data and use the component
+  </figcaption>
+
+  <DataSource
+    @src={{uri '/partition/namespace/dc-1/services'}}
+  as |source|>
+
+    <<%= classifiedModuleName %>
+
+    >
+    </<%= classifiedModuleName %>>
+
+  </DataSource>
+
+</figure>
+```
+
+## Attributes
+
+<!-- START component-docs:@attrs -->
+<!-- END component-docs:@attrs -->
+
+## Arguments
+
+<!-- START component-docs:@args -->
+<!-- END component-docs:@args -->
+
+## Slots
+
+<!-- START component-docs:@slots -->
+<!-- END component-docs:@slots -->
+
+## CSS Parts
+
+<!-- START component-docs:@cssparts -->
+<!-- END component-docs:@cssparts -->
+
+## CSS Properties
+
+<!-- START component-docs:@cssprops -->
+<!-- END component-docs:@cssprops -->
+
+## Contextual Components
+
+<!-- START component-docs:@components -->
+<!-- END component-docs:@components -->

ui/packages/consul-ui/blueprints/component/files/__root__/__templatepath__/__templatename__.hbs

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+{{yield}}

ui/packages/consul-ui/blueprints/component/index.js

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+module.exports = Object.assign(
+  require('ember-source/blueprints/component/index.js')
+);
+