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.
local test passed
addgroupcopy test passed
clone test passed
lowlevelcopy test passed