Add string position of tokens in token trees

[synga-nl]

This PR adds the position to the tokens in the token tree.

Example found in the following string:
I want to {{ try }} a more {% if advanced | length > 3 %}{{ variable }}{% endif %} template {% set unit = 2 %}{# This is a comment #}{{ variable_after_comment }}

  {
    type: 'raw',
    value: ' template ',
    position: { start: 82, end: 92 }
  },

This means the raw text template can be found between position 82 and 92 in the given template.

This is one of a series of PRs I want to make to parse through tokens and to act on these tokens. Like the NodeTraverser of Twig in PHP.

[willrowe]

This looks pretty good. I don't see any reason why it can't be merged, any thoughts @RobLoach?

[RobLoach]

Makes sense to me 👍

[synga-nl]

@willrowe or @RobLoach is it possible to create a minor version of this on NPM? That makes testing the package in other applications way more easy for the other PRs (token traverser).

[willrowe]

I was going to add this to the v1.17.0 milestone that I will be choosing fixes/features for in the next couple of weeks, so I'm not sure if we'd do a release for just this or not.