Allow use of "* * *" as delimiter for special variables

I have the following template:
```
---

project: dropdown(TEST1,TEST2,TEST3)
template_title: {{ date }} - {{ project }}
template_tags: TESTTAG, {{ project }}

---

# {{date}}
{{project}}:
```

If I switch to the wysiwyg editor and make some modification, then all the "---" get replaced with "* * *", breaking the variables in the template

The only installed plugins are templates and backup (probably not related to the issue)

I guess the easy way to fix this could be to allow "* * *" as the special variables delimiter [HERE](https://github.com/joplin/plugin-templates/blob/d4b319f10d0db5717d575d7654395ce54cfdda29/src/parser.ts#L173)