FEATURE: when rich-editor is enabled use Jetbrains Mono as code font (#32122)

This change forces consistency around code font in Discourse once the
new
rich-editor is enabled

- This means all code blocks in Discourse will render with this font
- Additionally the markdown mode composer will render with this font

Additionally we make 3 small adjustments:

1. We disable ligatures which conflict with typographer
2. We add support for custom ligature settings
3. We adjust down font size, cause 14px ends up matching visually with a
16px non monospace font, this change is already in place previously on
posts

Example:


![image](https://github.com/user-attachments/assets/eca0b544-f3a4-4172-b2af-b39a3c0208e7)

lib/stylesheet/importer.rb

@@ -64,9 +64,31 @@ module Stylesheet
 
         CSS
 
+      if SiteSetting.rich_editor
+        contents << <<~CSS
+          #{font_css(jetbrains_mono)}
+          #{render_font_special_properties(jetbrains_mono, "body")}
+          :root {
+            --d-font-family--monospace: #{jetbrains_mono[:stack]};
+          }
+
+        CSS
+      else
+        contents << <<~CSS
+          :root {
+            --d-font-family--monospace: ui-monospace, "Cascadia Mono", "Segoe UI Mono", "Liberation Mono", menlo, monaco, consolas, monospace;
+          }
+
+        CSS
+      end
+
       contents
     end
 
+    def jetbrains_mono
+      @@jetbrains_mono ||= DiscourseFonts.fonts.find { |f| f[:key] == "jet_brains_mono" }
+    end
+
     def wizard_fonts
       contents = +""
 
@@ -262,6 +284,12 @@ module Stylesheet
       else
         contents << "#{" " * indent}font-feature-settings: normal;\n"
       end
+
+      # avoiding adding normal to the CSS cause it is not needed in this case
+      if font[:font_variant_ligatures].present?
+        contents << "#{" " * indent}font-variant-ligatures: #{font[:font_variant_ligatures]};\n"
+      end
+
       contents
     end
   end