Improve template names and line numbers in error messages

[jg-rp]

While Python Liquid's error messages do contain template names and offending line numbers where possible, there is room for improvement, especially for expressions that span multiple lines.

The error message format implemented in LiquidScript seems more readable, and LiquidScript keeps track of the starting index of each token in template source text, rather than a line number, resulting in more accurate error messages.

[janwirth]

I consider chumsky / ariadne error reporting for parser to be the best out there right now:
https://github.com/zesterer/chumsky

[jg-rp]

Python Liquid2 includes more context info in exceptions and much improved error messages (example).

In time, if there's a demand, we'll implement something similar here too. This will require a major update to the scanner/lexer (needed for #136 as well), a redefinition of Token and a change to the base Error exception API.