.arcconfig

templater: drop extension point of engine classes (API)

I don't think this would ever be used by third-party extensions, as we've
heavily changed both the templater internals and the syntax since then.

The main reason of removing this API is that I want to move the parsing
function from the engine to the templater class so that we can peek keywords
and functions used in a user template. This change also removes reference
cycle between the templater and the engine.

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