chore: replace fs-extra with native Node.js fs

[roli-lpci]

Summary

• Replace fs-extra with native node:fs/promises and node:fs APIs
• All replacements available on supported Node >= 18
• Removes 1 runtime dependency (fs-extra + 3 transitive deps)

Follows the same pattern as #489 (globby → tinyglobby).

Changes

Source (1 file):

• index.js — fs.pathExists() → fs.access(), fs.outputFile() → fs.mkdir() + fs.writeFile()

Tests (10 files):

• test/helpers/clean.js — fs.emptyDir() → fs.rm() + fs.mkdir(), fs.remove() → fs.rm()
• test/helpers/env.js — fs.copy() → fs.copyFile(), fs.outputFile() → fs.writeFile()
• test/helpers/read.js — import swap only
• test/stdin.js, test/stdout.js — fs.createReadStream() → named import from node:fs
• test/ext.js, test/map.js — fs.pathExists() → fs.access()
• test/replace.js — fs.copy() → fs.copyFile()
• test/unchanged.js, test/watch.js — import swap only

package.json — removed fs-extra from dependencies

Replacement mapping

fs-extra	Native (Node >= 18)
pathExists()	access().then(() => true, () => false)
outputFile()	mkdir({ recursive: true }) + writeFile()
copy()	copyFile() (glob ensures file-only results)
emptyDir()	rm({ recursive, force }) + mkdir({ recursive })
remove()	rm({ recursive, force })
createReadStream()	Named import from node:fs

Testing

All 39 tests pass. ESLint and Prettier clean.