Cleanup codegen

[xoofx]

The codegen in Scriban has grown organically with some legacy constraints:

• AsyncCodeGen: Using Roslyn, requires to launch outside in a separate tool to disable async on Scriban and to re-generate them. This project also generates all the visitors/rewriters (that don't care about async codegen)
• DelegateCodeGen: Using Cecil. Might not be need anymore with the new delegate approach in Scriban 3.x or could be simplified.
• DocGen: Use NuDoq to extract the doc and read md files.

We should probably try to have at worst 2 codegen paths:

• One using source generators: e.g for visitors, delegates (so part of AsyncCodeGen and DelegateCodeGen)
• An external tool for the async generator part + the doc generation part