Set EasyMDE heading font-size to the same size as the resulting markdown (#24151)

Fix #23816

According to my personal experience, the EasyMDE is still useful when
writing a lot of contents, eg: the wiki page.

It's not difficult to improve its heading styles, so let's make it.

Before:

<img width="815" alt="image"
src="https://user-images.githubusercontent.com/2114189/232280943-9177f0bc-e380-426f-8588-20ff8d8e5293.png">

After:

<img width="538" alt="image"
src="https://user-images.githubusercontent.com/2114189/232280903-e8c476ee-f5b1-48fe-8a93-86fcd79680c3.png">

Author: wxiaoguang

web_src/css/editor-markdown.css

@@ -44,6 +44,31 @@
   max-height: calc(100vh - 200px);
 }
 
+/* use the same styles as markup/content.css */
+.combo-markdown-editor .CodeMirror-scroll .cm-header-1 {
+  font-size: 2em;
+}
+
+.combo-markdown-editor .CodeMirror-scroll .cm-header-2 {
+  font-size: 1.5em;
+}
+
+.combo-markdown-editor .CodeMirror-scroll .cm-header-3 {
+  font-size: 1.25em;
+}
+
+.combo-markdown-editor .CodeMirror-scroll .cm-header-4 {
+  font-size: 1em;
+}
+
+.combo-markdown-editor .CodeMirror-scroll .cm-header-5 {
+  font-size: 0.875em;
+}
+
+.combo-markdown-editor .CodeMirror-scroll .cm-header-6 {
+  font-size: 0.85em;
+}
+
 text-expander {
   display: block;
   position: relative;