Proposal: add a prepare() pass before run() pass

[sidvishnoi]

ReSpec has "prepare" code like this or this in many modules. My idea is to add a async prepare(conf) in such modules, and do a pass where all modules run their prepare(), followed by our existing run() pass.

Why?

• I was trying to reorder some plugins to make overall ReSpec appear to load faster (mainly create ToC before content below the fold). But some plugins add headings dynamically (not available until they run()), but ToC requires those headings (so we've a dependency chain making ToC render late). If we can generate those headings in a prepare, then we can easily generate ToC earlier (See this demo of impact: https://respec-run-toc-early.netlify.app).
• This would also help with https://github.com/w3c/respec/pull/2187 where we can keep mutations in prepare, and not make them top-level.

Thoughts? @saschanaz @marcoscaceres

[saschanaz]

This sounds good! I see no immediate negative point.

[marcoscaceres]

Like it! /me presses 👍 button.