add options for custom lunr Liquid and JS code (#1068)

This is a prototype for review and discussion. My use and testing of this PR is on top of 6d9d41359c. The changes are trival to rebase to `main` and I'm happy to do so if this prototype moves forward.


* Feature request details in linked issue, fixes just-the-docs/just-the-docs#1067
* I welcome feedback and all discussion
* A draft doc site of mine using this PR is at https://docs.hidale.com/

To use the prototype, the two include files need to be customized. Here are mine from the draft website https://github.com/diablodale/dp.docs/commit/9c0d836408af2e72dbce115a01ce6627137e66dd

Co-authored-by: Matt Wang <matt@matthewwang.me>

assets/js/just-the-docs.js

@@ -87,6 +87,7 @@ function initSearch() {
         this.metadataWhitelist = ['position']
 
         for (var i in docs) {
+          {% include lunr/custom-index.js %}
           this.add({
             id: i,
             title: docs[i].title,