[clangd] Introduce paragraph, the first part of new rendering structs

Summary:
Initial patch for new rendering structs in clangd.

Splitting implementation into smaller chunks, for a full view of the API see D71063.

Subscribers: ilya-biryukov, MaskRay, jkorous, arphaman, usaxena95, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D71248

Reviewers: sammccall