AST

[Andrew-Morozko]

Partial switch to AST:

• Plugins have the ability to return ast content (see stixview)
• Printers (to be renamed to formatters) handle custom nodes and incorrect nodes (i.e. raw html in pdf printer) and replace them with error messages

What's left to do:

• Plugins can return custom blocks and custom inline elements (an arbitrary protobuf message inside of the AST)
• Formatters can declare and expect custom protobuf messages to be returned to them inside the AST (i.e. RawLaTeX)
• Plugins can declare custom protobuf messages and provide custom RPC format functions for them (they would be called to translate plugin's protobuf message to the set of messages that could be used by the formatter)

I truly am sorry for another huge PR, even though I split this feature in two pieces it still got away from me 😭

[anasmuhmd]

Whew! Thank you! @Andrew-Morozko

[anasmuhmd]

Shouldn't this be c = repl.NextSibling() ?

[Andrew-Morozko]

It's intentional, added a comment and a limit on recursive replacements.

This allows replacer to be a pure function for replacement logic like
Node A needs to be replaced by CustomRendererA
Node B needs to be replaced by CustomRendererB

CustomRendererA may replace Node A with Node B, or include Node B in one of the children. We don't know what custom renderers may do. Speaking of this, added the ability to return errors from replacers, useful when remote plugins are in play.