.arcconfig

templater: convert resources to a table of callables for future extension

I'm going to add a full templating support to the annotate command. As the
annotate is a filectx-oriented command, we'll  need a way to look up a ctx
from a bounded fctx only when necessary.

This is the minimal change to support that. I'm thinking of defining a proper
interface to look up template resources to fix other issues, but that isn't
ready yet.

(Changes from V1: just updated tests and patch descriptions.)

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