docs: update readme and contributing from pnpm dev to pnpm watch

[raulrusu88]

while I doing the codebase setup locally, I have found that pnpm dev is not defined in any package.json file, but was renamed to watch

and added to the README Type-Checking command.

Svelte 5 rewrite

Please note that the Svelte codebase is currently being rewritten for Svelte 5. Changes should target Svelte 5, which lives on the default branch (main).

If your PR concerns Svelte 4 (including updates to svelte.dev.docs), please ensure the base branch is svelte-4 and not main.

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 and linting

• Run the tests with pnpm test and lint the project with pnpm lint

[benmccann]

I guess the question is if we should rename the command or update the docs. I'll defer to the others on that. We do use dev in SvelteKit, but I'm not sure if folks feel it's different in this case where we don't have a server

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[Rich-Harris]

Thank you — it turns out there's a whole bunch of inaccurate information in the contributor guide/readme, so I'm going to close this in favour of #9673