Add monospace toggle button to textarea (#24034)

- Add new button to textarea to switch font. State is persisted in
localStorage.
- Change markdown-switch-easymde button from `<span>` to `<button>`
- Slightly increased monospace font globally by 5% as I think it fits
better.

For hover effect on these buttons I'm deferring to
https://github.com/go-gitea/gitea/pull/23896.


![](https://user-images.githubusercontent.com/115237/230948526-ecf8d730-0c69-4a8e-a1a5-1e5e079c754d.gif)

---------

Co-authored-by: delvh <dev.lh@web.de>

web_src/css/editor-markdown.css

@@ -24,6 +24,7 @@
   user-select: none;
   padding: 5px;
   cursor: pointer;
+  color: var(--color-text);
 }
 
 .combo-markdown-editor .markdown-toolbar-button:hover {

web_src/css/helpers.css

@@ -29,7 +29,7 @@
 
 .gt-mono {
   font-family: var(--fonts-monospace) !important;
-  font-size: .9em !important; /* compensate for monospace fonts being usually slightly larger */
+  font-size: .95em !important; /* compensate for monospace fonts being usually slightly larger */
 }
 
 .gt-bold { font-weight: 600 !important; }