feat: String templates

[saig0]

Description

Add support for string templates. Align with the Feelers library.

• Template syntax: \```template```\ (tripple backtick)
• Inline expressions {{variable or express}}
• Conditional sections {{#if condition}}here{{/if}}
• Loops {{#loop listValue}} Item: {{this}} {{/loop}}

Example

Template:

```Hello {{user.name}},

We received your order (ID: `{{order.id}}`) with the following items:
{{#loop order.items}}
    - {{this.quantity}}x {{this.name}}
{{/loop}}

Greetings {{department.name}}```

Context:

{
  "user": {
    "name": "Zee"
  },
  "order": {
    "id": "c-8",
    "items": [
      {
        "name": "Helmet",
        "quantity": 1
      },
      {
        "name": "Flag",
        "quantity": 1
      },
      {
        "name": "Oxygen tank",
        "quantity": 3
      }
    ]
  },
  "department": {
    "name": "Camunda"
  }
}

Result:

Hello Zee,

We received your order (ID: `c-8`) with the following items:
    - 1x Helmet
    - 1x Flag
    - 3x Oxygen tank

Greetings Camunda

Related issues

closes #392

[nikku]

@saig0 before we merge that, are we clear on the actual whitespace semantics, inside a tripple backtick block? Especially as this is a construct that can be arbitrarily nested, this becomes relevant:

{ 
  body: ```
    HELLO {{ user.name }}! How are you?
    
    Greetings {{ us }}
    ```
}

What is the value of body? Is it

........HELLO Nico! How are you?
........
........Greetings Camunda
........

Or are we "smart enough", and remove the leading whitespace (every line) as well as leading and trailing empty lines:

HELLO Nico! How are you?

Greetings Camunda