amp-analytics: Shared interpolation & ampState() URL variable

• Extract amp-analytics variable interpolation parsing (${var}) so it can be shared with another extension
• Implement ${ampState(foo.bar)} variable that reads from amp-bind state

/cc @lannka

[lannka]

The variable code is here

expandTemplate is the main function. Right now you can ignore any filter related code as they are dead code and @calebcordry is cleaning up.

To be clear, this VariableService code is to provide recursive var substitution to JSON config:

{
  vars: {
    varA: "123",
    varB: "${varA}"
  },

  requests: {
     a: "${varA}&${varB}"
  }
}

if you do expandTemplate(requests.a), it will return a promise of 123&123.

Another way to do similar thing is to use urlReplacementService, of which the code is already in core. To get what you want, you will add an AMP_STATE macro to url-replacements-impl.js, and use it like:

{
  requests: {
   a: "AMP_STATE(x)"
  }
}

I think urlReplacementService might be a better choice at this point, despite its name does not sound relevant.

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