[feat] Track unsaved files in REPL

[joshnuss]

Fixes sveltejs/svelte-repl#117

If a user has unsaved changes in the REPL and navigates away, all changes are lost. This has bit me quite a few times.

This PR adds a warning "There are unsaved changes" to prevent any the data loss.

This change is in conjunction with sveltejs/svelte-repl#156.

I'm opening this PR so that we can discuss if this method is correct or if we should a different path.

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
• 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

[benmccann]

The site was migrated to SvelteKit, so this would need to be rebased

I'm curious your thoughts between this and #6251 since it seems like we would only need one of the two functionalities

[bluwy]

I think this feels better (and safer) than #6251. This would need a rebase though.

[tanhauhau]

i've rebased the branch, looks good now. Need to publish a new version of the REPL after sveltejs/svelte-repl#156, before merging this branch

[benmccann]

I pushed a new version with updated REPL, but the site won't load now. Hopefully Vite 2.7 will fix this or at least give a better error message. I can't easily test with pnpm overrides because I'm not sure how to update our crazy codemirror setup in svelte.config.js to be compatible with pnpm. Vite 2.7 should hopefully come out on Monday, then we can update SvelteKit and try this again

[benmccann]

I tracked down the error to an issue in the REPL change: sveltejs/svelte-repl#188