Add/expose a preprocessor callback in `config.ld`

[Elv13]

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(this issue split of a discussion in a ticket regarding removing the sandbox and supporting modules)

In AwesomeWM, I "abuse" of some features to get access to the processed AST tree (items tree) in config.ld. This allows to both inject new properties to the item and create indices.

Right now the way I do this is very hacky. Given ldoc was unmaintained, it was "fine", but if development restarts, I fear that abusing of many undocumented hacks will just break in every single releases. Please provide an official API to access the AST before the template is rendered from config.ld before making a new release. I don't mind if my hacks break as long as there is an official API replace them.

Right now, I don't use it as much as I could because adding all of that business logic makes config.ld very large (see #333), but the possibilities are endless once you have that. This plus #333 would allow proper ldoc plugins to add features without changes to the core code.