[JohnathonKoster]

This PR adds opt-in instrumentation to Antlers. When enabled, comments are injected into the rendered HTML that point back to the template expression that produced each bit of output

What the output looks like

Given this template:

<section class="hero">
  <h1>{{ title }}</h1>
  <input value="{{ title }}">
  {{ if show }}<p>{{ body }}</p>{{ /if }}
</section>

you get this (payloads trimmed):

<section class="hero">
  <h1><!-- antlers:start eyJ0eXBl… -->Hello<!-- antlers:end 1 --></h1>
  <input value="Hello">
  <!-- antlers:start eyJ0eXBl… --><p><!-- antlers:start eyJ0eXBl… -->World<!-- antlers:end 3 --></p><!-- antlers:end 2 -->
</section>

The start marker payload is base64 JSON:

{
  "type": "single",
  "engine": "antlers",
  "expression": "title",
  "line": 2,
  "start": 29,
  "end": 40,
  "context": "element-content",
  "element": "h1",
  "view": "page.antlers.html",
  "id": 1
}

Docs/API

Docs and API are still a work in progress; most of the API surface area has been marked internal to discourage third-party use (for now) and to allow freedom to change things without worrying about backwards compatibility, and also not sit on this feature set until its "perfect" 🙂