Interactive custom model editor improvements

Continuing from graphhopper/graphhopper#2239 the following things could/should be improved:

- [x] allow negating areas using `== false`
- [x] check syntax for areas section (geojson)
- [x] support boolean encoded values without `==/!= true/false`? This is important especially for, e.g. `if: true, limit to: 100`
- [x] ~~auto-convert tabs to spaces, visualize space characters~~ -> no longer needed since we use json now
- [ ] fix missing auto completion for e.g. `road_class ==| MOTORWAY && in_berlin`
- [x] support negation operator `!`
- [ ] support heading penalty
- [x] ~~enable soft wraps?~~ disable line numbers, disable error icons in left gutter
- [ ] full screen for editor?
- [ ] show 'no suggestions' popup instead of not showing anything when there are no suggestions?
- [ ] auto-complete some values, like empty lists for `speed/priority`, `type: Polygon/Feature`, `else: ""` etc.
- [ ] support areas defined in server-side profiles, does this even work?
- [x] auto-complete for ~~YAML~~ JSON, especially speed/priority/areas/distance_influence/limit_to/multiply_by would be nice
- [x] give better feedback for hard-to-find indentation errors (maybe use match brackets api to find closing bracket) -> not as critical for JSON anymore, we use match brackets though
- [x] enable close-brackets: https://codemirror.net/demo/closebrackets.html
- [x] catch ~~YAML~~ JSON syntax errors that aren't caught currently, e.g. `{}[]`.
- [ ] maybe trigger autocomplete automatically without pressing Ctrl+Space? At least in certain cases? This way maybe users not familiar with auto-complete get to know it more easily.
- [ ] probably related to above: update (or close?) auto-complete popup when user keeps typing after it was opened?
- [x] auto-complete for numeric encoded values? maybe just show a hint saying that one needs to enter a number? show "type a number" as suggestion and insert nothing when user presses enter
- [x] tweak CM 'mode' especially indentation is not useful at all, e.g. for almost all cases it does the opposite of what we want, see here: https://codemirror.net/doc/manual.html#modeapi -> not as critical for json, and we use a black font for now since the default (red) seems a bit too much
- [ ] simplify error messages (now that they are shown at their origin) ?
- [ ] maybe use tooltips to provide some documentation, e.g. for speed/priority/distance_influence
- [ ] make it easy to translate error messages? but first make them as simple as possible.
- [x] maybe use code mirror bracket matching
- [ ] adjust colors, theme and styles
- [ ] check code mirror options to see if we need anything besides the defaults, maybe customize the 'mode'?
- [x] ~~handle YAML multiline syntax like `>+` and `|`... https://stackoverflow.com/a/21699210~~ we use JSON now
- [ ] dark theme and VIM keybindings (just kidding...)