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

.editorconfig

@@ -0,0 +1,19 @@
+root = true
+
+[*]
+charset = utf-8
+end_of_line = lf
+insert_final_newline = true
+indent_style = space
+indent_size = 2
+trim_trailing_whitespace = true
+
+[*.java]
+indent_size = 4
+
+[*.md]
+max_line_length = off
+trim_trailing_whitespace = false
+
+[*.puml]
+insert_final_newline = false