chore: markdown warnings

[Rich-Harris]

This is #11294 but for warnings. As before, the warnings themselves (codes and messages) could use a bit of tidying up but I didn't want to derail the PR — better to get the foundations laid first.

Will self-merge once green to unblock other work, as it should be uncontroversial (since it's using the same mechanism as before)

[changeset-bot]

⚠️ No Changeset found

Latest commit: 2f5b26e

Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.

This PR includes no changesets

When changesets are added to this PR, you'll see the packages that this PR includes changesets for and the associated semver types

Click here to learn what changesets are, and how to add one.

Click here if you're a maintainer who wants to add a changeset to this PR

[Rich-Harris]

Added the warning messages as JSDoc annotations so we can easily see them inside the codebase by hovering over the calls to e.whatever(...) and w.whatever(...)