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>

options/locale/locale_en-US.ini

@@ -133,6 +133,8 @@ buttons.list.task.tooltip = Add a list of tasks
 buttons.mention.tooltip = Mention a user or team
 buttons.ref.tooltip = Reference an issue or pull request
 buttons.switch_to_legacy.tooltip = Use the legacy editor instead
+buttons.enable_monospace_font = Enable monospace font
+buttons.disable_monospace_font = Disable monospace font
 
 [filter]
 string.asc = A - Z