Add source map for built HTML files

Liquid can be thought of as an HTML compiler. This is similar to SASS CSS compiler or a Javascript minifier.

When debugging a compiled project, it can be helpful to know the origin of each line of the output. For example, if a built html project using liquid (maybe through Jekyll) may contain this HTML error: <img src="url"> (it is missing an alt tag). Normally it would require a human to figure out which include file this error originated from.

In modern times testing is usually done automatically (maybe Travis CI running against a website published on GitHub). It will be nice if the test errors, which are based on the built HTML, can be automatically traced back into the original source code, the Liquid files and partials includes.

There is a standard for this already and it is called Source Maps. This technology is widely used for CSS, JS and Solidity compilation as well as other languages I'm sure.

I believe this is the authoritative documentation:
https://docs.google.com/document/d/1U1RGAehQwRypUTovF1KRlpiOFze0b-_2gc6fAH0KY0k/edit

An implementing library: https://github.com/mozilla/source-map

Please consider to implement Source Maps for Liquid.

Start by reviewing the issue's Source Maps specification and the linked mozilla/source-map library, then trace how Liquid builds HTML and handles includes and partials. Done means generated HTML includes usable mappings back to the originating Liquid files and partials, with the behavior verified by tests.