frontend 2.0: initial version

- auto refresh function
- light and dark theme
- monaco editor (vscode) with "apex" as syntax highlighting language
  * apex seems to work quite fine (better than no highlighting)
  * future possibility: own plantuml language syntax support
  * future possibility: autocomplete (to much work but maybe partial)
    - implemented example for `!theme ...`
    - implemented example for `<&icon>`
  * future possibility: code validation
    - implemented example for `@start...` and `@end...`:
      * should be the first or last command
      * should be of the some type (e.g. `@startyaml` and @endyaml)
      * should be used exactly once per document/diagram
- editor and preview is splitable into two windows like the
  "Extract window" functionality on
  (plantuml.com)[https://www.plantuml.com/plantuml]
- multi index / multi paging diagram support
- diagram can be displayed/rended as PNG, SVG, ASCII Art or PDF
- Ctrl+s download the PlantUML Code as code file (diagram.puml)
- Ctrl+, opens the settings and Esc closes the settings

.vscode/settings.json

@@ -2,21 +2,23 @@
     "java.configuration.updateBuildConfiguration": "automatic",
     "cSpell.words": [
         "Arnaud",
+        "buildx",
         "ditaa",
         "endditaa",
         "enduml",
         "epstext",
         "etag",
+        "ghaction",
         "Lalloni",
+        "monaco",
         "plantuml",
         "Roques",
         "servlet",
         "servlets",
         "startditaa",
         "startuml",
-        "utxt",
-        "ghaction",
-        "buildx"
+        "undock",
+        "utxt"
     ],
     "cSpell.allowCompoundWords": true
 }