build: use tsdown + isolated declarations

[danielroe]

📚 Description

this is a PR experimenting with tsdown - for a couple of reasons:

1. moving away from jiti stub mode for CLI allows more accurate performance testing
2. this fixes some build failures in perf(info): migrate from clipboardy to copy-paste #1087

[pkg-pr-new]

nuxt-cli-playground

npm i https://pkg.pr.new/create-nuxt@1095

npm i https://pkg.pr.new/nuxi@1095

npm i https://pkg.pr.new/@nuxt/cli@1095

commit: 1ae00e9

[github-actions]

📦 Bundle Size Comparison

📉 nuxi

Metric	Base	Head	Diff
Rendered	5083.62 KB	5020.76 KB	-62.86 KB (-1.24%)

📉 nuxt-cli

Metric	Base	Head	Diff
Rendered	108.86 KB	100.95 KB	-7.91 KB (-7.27%)

📈 create-nuxt

Metric	Base	Head	Diff
Rendered	2182.16 KB	2321.85 KB	+139.69 KB (+6.40%)

[codspeed-hq]

CodSpeed Performance Report

Merging #1095 will not alter performance

_{Comparing build/tsdown (1ae00e9) with main (0826403)}

Summary

✅ 2 untouched