Feature: handlebars frontend

[djc]

In order to make Askama more accessible to folks used to handlebars templates (https://crates.io/crates/handlebars is likely the most Rust popular template engine), it would be awesome if Askama could parse (a sensible subset of) handlebars templates and compile/transpile it to the Askama AST, enabling people to use handlebars templates with Askama codegen.

[harishletsgo]

@djc Handlebars templates are more frontend based than lets say Jinja2. I'd say the functional features that Jinja offers outweighs what Handlebars does and offers. I see a direct standoff between Tera and Askama. I frankly skipped Handlebars because of its low synergy with the server side, implemented Tera but it was too slow and painful. Askama is a great improvement over what Tera could be but cannot.