chore: update PR template (#9397)

* chore: update PR template

* Update .github/PULL_REQUEST_TEMPLATE.md

---------

Co-authored-by: Rich Harris <richard.a.harris@gmail.com>

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,6 +1,9 @@
-## Svelte compiler rewrite
+## Svelte 5 rewrite
+
+Please note that [the Svelte codebase is currently being rewritten for Svelte 5](https://svelte.dev/blog/runes). Changes should target Svelte 5, which lives on the default branch (`main`).
+
+If your PR concerns Svelte 4 (including updates to [svelte.dev.docs](https://svelte.dev/docs)), please ensure the base branch is `svelte-4` and not `main`.
 
-Please note that [the Svelte codebase is currently being rewritten](https://svelte.dev/blog/runes). Thus, it's best to hold off on new features or refactorings for the time being.
 
 ### Before submitting the PR, please make sure you do the following