docs: correct stale in-progress labels for the VS Code and Vim/Neovim extensions

[otnc]

Summary

• Fix bug(s)
• New feature(s)
• Others

While checking what's left on the roadmap, found that DESIGN.md chapter 13's status labels for both editor extensions were stale: "VS Code extension ... in progress" and "Vim/Neovim plugin ... highlighting done, everything past that still open", even though each entry's own body already describes every planned feature as done (folding, jump-to-definition, undefined-variable detection, inline missing/stale/machine indicators for both; Neovim-only for the last two on the Vim side, which need vim.diagnostic/vim.system). Fixed the two labels to say "done" and note the Neovim-only bit, in both locales.

Docs only.

Why?

The headline contradicted its own body and made both extensions look unfinished when they aren't.

Related issue(s)

closes

Check

• Check if duplicate PR(s) already exist.
• Read CONTRIBUTING.md (rules)
• Run npm run check (lint & format)
• Run npm run typecheck
• Run npm run test (functions test)
• Run npm run build (dist/)
• (optional) Run npm run test:coverage and check nothing new went uncovered