Extract CodeMirror styles (#17960)

- Extract CodeMirror-related styles to separate files
- Generalize CodeMirror styles where possible
- Improve fullscreen and side-by-side mode for dark theme

Co-authored-by: 6543 <6543@obermui.de>

web_src/less/_repository.less

@@ -2040,26 +2040,8 @@
     }
 
     &.new {
-      .CodeMirror {
-        .CodeMirror-code {
-          font-family: var(--fonts-monospace);
-
-          .cm-comment {
-            background: inherit;
-          }
-        }
-      }
-
-      .editor-preview {
-        background-color: white;
-      }
-
       .ui.attached.tabular.menu.previewtabs {
         margin-bottom: 15px;
-
-        & + .field .editor-toolbar:not(.fullscreen) a.fa-eye {
-          display: none;
-        }
       }
     }