always include source map in ts-preprocess of internal svelte compile

[jasonlyu123]

svelte-preprocess merge the compilerOption with the parsed config file
https://github.com/sveltejs/svelte-preprocess/blob/36d6421db847f8886ebad4c0e8c77671a9a4e3f9/src/transformers/typescript.ts#L69
So it won't override other user-defined configs. Although the problem mentioned here #489 (comment) here still exists, this would at least make the set up a little bit easier.

[dummdidumm]

This will only fix errors for people who do not have a svelte.config.js inside their workspace, because it's only a fallback. But as you say, in these cases it will make setup easier 😄