[fix] attribute values are incorrectly escaped during ssr

[mrkishi]

Fixes #7327 and a related security issue.

Attribute values were being JSON.stringifyed after being escaped, resulting in escaped characters like \\ and \n to show up in the HTML. The browser doesn't JSON.parse attribute values, so the attributes end up with the wrong value.

Succinctly: <el attr={'\\'}> renders <el attr="\\"> gives el.getAttribute('attr') === '\\\\'.

The security issue is that objects, on the other hand, were rendered directly to attribute values as unescaped strings. This means an object with a custom toString() can result in raw html injection.

See modified tests for more details.

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

• It's really useful if your PR references an issue where it is discussed ahead of time. In many cases, features are absent for a reason. For large changes, please create an RFC: https://github.com/sveltejs/rfcs
• Prefix your PR title with [feat], [fix], [chore], or [docs].
• This message body should clearly illustrate what problems it solves.
• Ideally, include a test that fails without this PR but passes with it.

Tests

• Run the tests with npm test and lint the project with npm run lint