fix: fix accumulated stacks in error overlay

[hi-ogawa]

Description

This looks like a regression of #15852. Since template element became a singleton, when I show the error overlay programatically with server.hot.send, it accumulates the messages in .stack due to the mutation here:

vite/packages/vite/src/client/overlay.ts

Line 269 in 5f0e6a3

el.appendChild(document.createTextNode(frag))

The same test case looks like this in main:

Show screenshot

which now looks like this:

The background is that, I'm currently trying runtime error overlay ideas like #6274 and then I just spotted this odd behavior. Normally, the error overlay seems to happen after full client reload, so template is fresh, but when trying to show overlay multiple times, the stacks keep accumulating.

I suppose calling createTemplate every time wouldn't hurt much, so I simply removed template singleton. Before the PR #15852, it was innerHTML assignment, so I think it was making a fresh copy similarly.