feat: implement message overloads

[Rich-Harris]

See #11305.

This makes it possible for an error/warning message to have multiple variants, to satisfy the common pattern of appending 'Did you mean %suggestion%?' with incorrect bindings etc, without needing multiple codes for what is essentially the same issue.

It also adds the ability have a separate prose section, which isn't currently used anywhere but can be in future:

## error_code

> Incorrect thing %thing%

> Incorrect thing %thing%. Did you mean %suggestion%?

Here is a longer explanation of this thing.

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• 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
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• 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

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