Specify Node version requirement

[GrygrFlzr]

Closes #354.

• Accept 12.17.0+
• Accept 14.13.0+
• Decline 13.x

On unsupported versions, the following message will appear when attempting to run e.g. pnpm test:

$ pnpm test
 WARN  Unsupported engine: wanted: {"node":">= 12.17.0 < 13.0.0 || >= 14.13.0"} (current: {"node":"v14.12.0","pnpm":"5.18.3"})

[Rich-Harris]

I think we can fix #354 without restricting SvelteKit to specific Node versions: #465

I'm confused by these Windows test failures though, they seem to be happening on all PRs and I've no clue what could be causing it. Any ideas?

[GrygrFlzr]

Windows test failures will not be fixed until #460 is fixed, which requires a change in Vite. I've detailed the bug and the offending line over at the vite repo (vitejs/vite#2422), but I haven't yet come up with a fix that doesn't cause half the tests to fail.

[Rich-Harris]

Oof. Thanks for investigating, will keep an eye on that issue