fix(legacy): fix conflict with the modern build on css emitting

[haoqunjiang]

Description

Fixes #3296
Fixes #5325
Supersedes #3317

The asset emitting conflict may also exist for other types of assets,
but let's fix the CSS one first.

The conflict here is due to the hasEmitted flag that was originally
intended to avoid duplicated CSS for multiple output formats
6bce108#diff-2cfbd4f4d8c32727cd8e1a561cffbde0b384a3ce0789340440e144f9d64c10f6R262-R263

When the legacy plugin is used, the flag was set to true for the
emitted CSS of the legacy bundle.
But the legacy plugin would remove all its emitted assets later to avoid
duplication.

vite/packages/plugin-legacy/index.js

Lines 444 to 450 in 3fb4118

	if (isLegacyBundle(bundle, opts)) {
	// avoid emitting duplicate assets
	for (const name in bundle) {
	if (bundle[name].type === 'asset') {
	delete bundle[name]
	}
	}

So this logic results in no CSS to be actually emitted.

In this PR, I used a __vite_skip_asset_emit__ flag to prevent the
CSS generateBundle from executing for the legacy build.
If other asset emitting plugins encounter similar issues, this flag
can be reused.

Additional context

• Bug fix
• New Feature
• Documentation update
• Other

Before submitting the PR, please make sure you do the following

• Read the Contributing Guidelines.
• Read the Pull Request Guidelines and follow the Commit Convention.
• Check that there isn't already a PR that solves the problem the same way to avoid creating a duplicate.
• Provide a description in this PR that addresses what the PR is solving, or reference the issue that it solves (e.g. fixes #123).
• Ideally, include relevant tests that fail without this PR but pass with it.