Allow for larger indentation in the CodePane

[kale-stew]

Description

This PR allows for an indentSize prop to be supplied to the CodePane. Feedback welcome!

Type of Change

• New feature (non-breaking change which adds functionality)
• This change requires a documentation update

[ryan-roemer]

This doesn't fix #898

Here's a temp diff (don't commit this because this file is built from examples/js/index.js):

diff --git a/examples/one-page.html b/examples/one-page.html
index 96a9932..bfe15fe 100644
--- a/examples/one-page.html
+++ b/examples/one-page.html
@@ -13,8 +13,8 @@
     <script src="https://unpkg.com/[email protected]/umd/react-dom.production.min.js"></script>
     <script src="https://unpkg.com/[email protected]/umd/react-is.production.min.js"></script>
     <script src="https://unpkg.com/[email protected]/prop-types.min.js"></script>
-    <script src="https://unpkg.com/spectacle@^6/dist/spectacle.min.js"></script>
-    <!-- <script src="../dist/spectacle.js"></script> -->
+    <!-- <script src="https://unpkg.com/spectacle@^6/dist/spectacle.min.js"></script> -->
+    <script src="../dist/spectacle.js"></script>
 
     <script type="module">
       const {
@@ -237,6 +237,9 @@
             \`\`\`jsx
             const evenCooler = "is that you can do code in Markdown";
             // You can even specify the syntax type!
+            if (foo) {
+              bar();
+            }
             \`\`\`
 
             ### A slide can have multiple code blocks too.

[ryan-roemer]

By contrast, notice in a normal non-MD codeblock in the same presentation indentation works fine:

[carloskelly13]

This looks good to me.

[kale-stew]

Going to address the markdown code block issue in another PR! While this one doesn't close the originally linked issue, I think this is still a useful feature to introduce.