include NGINX variables in the dataset

[ryepup]

Is your feature request related to a problem? Please describe

Many NGINX modules define variables (e.g. $binary_remote_addr), and it would be great to have those in the reference-lib data.

Describe the solution you'd like

Either:

• variables are visible via the getDirectives return value, with a flag indicating if it's a variable or not
• variables are visible via a new getVariables function

Additional context

Variables seem to be defined with this pattern in the module files:

<section id="variables">
  <list type="tag">
  <tag-name><var>$VARIABLE_NAME</var></tag-name>
  <tag-desc>$DOCUMENTATION</tag-desc>
  </list>
</section>