The `json_ui.cc` file contained both data-parsing and UI-related
code. This commit moves the JSON parsing code to its own `json.cc`
file, to separate concerns, make compilation faster when changes are
made to either UI or parsing code, and make the parsing code more
accessible to fuzzers.
The signature of the following function:
```
auto parse_json(StringView json);
```
was changed to:
```
JsonResult parse_json(StringView json);
```
to avoid `auto` deduction issues at compile-time.
