[fix] update preprocessor types

[bluwy]

Fix preprocessor types:

• Allow Preprocessor to return void
• Make MarkupPreprocessor filename arg optional (always has been optional)

Refactor src/compiler/preprocess/index.ts too ensuring filename is nullable.

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Tests

• Run the tests with npm test and lint the project with npm run lint

[dummdidumm]

Why the void return type? Was that ever allowed, or when is this useful? The other things look fine 👍

[bluwy]

Yeah returning nothing for preprocessors were allowed for a long time. From the code, when we run preprocess, it would check whether we return something, otherwise bail out:

svelte/src/compiler/preprocess/index.ts

Line 188 in 6cc98a4

if (processed) {

svelte/src/compiler/preprocess/index.ts

Line 170 in 6cc98a4

if (!processed) return no_change();

Returning nothing is often used to early out on preprocessing due to certain fast checks, plus it would give a small perf boost too internally.

Another example in vite-plugin-svelte, we had to use ts-expect-error to be able to suppress typescript returning void: code

[dummdidumm]

Makes sense, thanks!