feat(create-docusaurus): add nub package manager support

[colinhacks]

Pre-flight checklist

• I have read the Contributing Guidelines on pull requests.
• If this is a code change: I have written unit tests and/or added dogfooding pages to fully verify the new behavior.
• If this is a new API or substantial change: the PR has an accompanying issue (closes #0000) and the maintainers have approved on my working plan.

Motivation

Adds nub as a selectable package manager in create-docusaurus, alongside npm/yarn/pnpm/bun. nub is pnpm-CLI-compatible and writes a nub.lock lockfile.

• constants.ts — add nub: 'nub.lock' to LockfileNames. Everything else derives from this map: lockfile detection, the --package-manager nub flag, the interactive prompt, and the PackageManager type all pick nub up automatically.
• utils.ts — the script tips relied on npm/yarn/pnpm/bun's implicit start shortcut (e.g. pnpm start). nub has no implicit script shortcut — nub start is an error — so every tip for nub is explicit: nub run start, nub run build, and so on. The run-prefix logic moved into a small packageManagerScriptCommand helper; output for the existing package managers is unchanged.
• commands.ts — nub uses the bun-style nub install and colors from the terminal / FORCE_COLOR; it does not accept the space-separated --color always form the generic branch passes.

Test Plan

• Added unit tests for packageManagerScriptCommand: nub gets an explicit run on every script; npm/bun keep start bare but prefix run elsewhere; yarn/pnpm never prefix run.
• Verified against nub 0.4.13: nub.lock lockfile detection, user-agent detection (npm_config_user_agent is nub/0.4.13 npm/? node/…, matched by the existing startsWith), nub --version for the availability prompt, nub install succeeds, and nub install --color always fails (the reason for the bun-style branch).

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[slorber]

Good start but there are many other places that need to be modified in this CLI to properly bring support (types, cli help option description, our docs and maybe more)

Also you may want to add nub as an option in our docs tabs, and for that you need a PR in npm-to-yarn converters first, see for example Deno recently added: bgub/npm-to-yarn#68

[colinhacks]

Thanks! Addressed:

• CLI help + docs: the --package-manager help text (bin/index.js) and the create-docusaurus API reference now list nub.
• Types: PackageManager is derived from LockfileNames (which this PR extends), so nub is already first-class across the type, validation, prompt, and detection — those two enumerations were the only remaining spots.
• Docs tabs: opened feat: add nub support bgub/npm-to-yarn#69 adding nub (mirroring the recent Deno addition). Once it's released and picked up here, I'll add the nub npm2yarn tab.

Let me know if anything else is missing.