Template visitor

[lucica28]

This pull request aims to increase the flexibility of the Visitor class so that it can be used with other AST families than ASTCodegen and at the same time make minimum changes to existing code.

[dlang-bot]

Thanks for your pull request and interest in making D better, @lucica28! We are looking forward to reviewing it, and you should be hearing from a maintainer soon. Please verify that your PR follows this checklist:

• My PR is fully covered with tests (you can see the coverage diff by visiting the details link of the codecov check)
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• I have provided a detailed rationale explaining my changes
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Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub run digger -- build "master + dmd#14424"

[RazvanN7]

@ibuclaw Is this solution acceptable?

[ibuclaw]

Where's the header update?

[RazvanN7]

On a second thought, this actually does not seem to fix anything. The general idea would be to template the hdrgen module too so that we can bundle it with the parsing library.