Export SourceMap types from root typings

[jridgewell]

A few project are using internal type files to get our RawSorceMap and DecodedSorceMap types:

• https://github.com/sveltejs/svelte/blob/ce3a5791258ec6ecf8c1ea022cb871afe805a45c/src/compiler/utils/mapped_code.ts#L1
• https://github.com/vitejs/vite/pull/2441/files#diff-6d149ac9706cd508b52db0c059a0f01d670620a9a5a73cd7b122ce62b6b69471R12

[jridgewell]

Closed by 6120a62.