[feat] add enableSourcemap option

[bluwy]

Supersedes #6744
Part of the fix for sveltejs/kit#720

This allows us to control the generation of sourcemaps for compile. This is non-breaking. Implementation-wise is quite simple and doesn't provide too much perf boost. The idea is to achieve #6744 but in a more generic way.

Option type:

{
  enableSourcemap: boolean | { js: boolean; css: boolean; } // default true
}

Note: This new option was discussed in the last maintainer's meeting (suggested by @lukeed), the plan is to have vite-plugin-svelte disable CSS sourcemaps for SSR only to fix sveltejs/kit#720.

Before submitting the PR, please make sure you do the following

• 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

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

[Conduitry]

Passing in strings to enable just CSS or just JS sourcemaps feels like a weird API to me. I think I'd prefer passing either a boolean (which controls everything) or an object with css and js keys which can have boolean values.

[bluwy]

Passing in strings to enable just CSS or just JS sourcemaps feels like a weird API to me. I think I'd prefer passing either a boolean (which controls everything) or an object with css and js keys which can have boolean values.

That was actually the initial proposal and I derailed from there 😅 Updated with the boolean | { js: boolean; css: boolean; } type.