[pseudo] Add `clang-pseudo -html-forest=<output.html>`, an HTML forest browser

It generates a standalone HTML file with all needed JS/CSS embedded.
This allows navigating the tree both with a tree widget and in the code,
inspecting nodes, and selecting ambiguous alternatives.

Demo: https://htmlpreview.github.io/?https://gist.githubusercontent.com/sam-mccall/03882f7499d293196594e8a50599a503/raw/ASTSignals.cpp.html

Differential Revision: https://reviews.llvm.org/D130004