feat(legacy): add rolldown-vite support

[sapphi-red]

Description

This PR adds rolldown-vite support to plugin-legacy.

This PR achieves that by downloading a built package that is published to pkg.pr.new on rolldown-vite repo and vendoring + loading that in the package.

The upsides of this approach are:

• the tests for rolldown-vite + plugin-legacy is done on rolldown-vite side and can be develeoped easier than having the source code of plugin-legacy for rolldown-vite in this repository
• users won't need to switch the package to a different one

The downsides of this approach are:

• the package size and download size is bigger due to some duplicated code and dependencies only used for rolldown-vite support. that said, rolldown-vite already depends on babel which is huge, so I think this is not a problem
• exports other than viteLegacyPlugin/default should be synced between the normal plugin-legacy and the one for rolldown-vite support. this limitation should be fine.

To update the version for rolldown-vite support, we can update this line:

vite/packages/plugin-legacy/tsdown.config.ts

Lines 6 to 8 in 20b727b

	// for rolldown-vite v7.0.9, check https://pkg.pr.new/~/vitejs/rolldown-vite
	const PLUGIN_LEGACY_FOR_ROLLDOWN_VITE =
	'https://pkg.pr.new/vitejs/rolldown-vite/@vitejs/plugin-legacy@5c52bbc'

[hi-ogawa]

Looks good to me!