feat: add option to preserve unused css selectors

[Maytha8]

Hello Svelte contributors,

This PR proposes to add a compile option, preserveUnusedSelectors, that keeps unused stylesheet selectors when set to true. It is set to false by default, which means it does not affect previous behaviour and is not a breaking change. I've also written the preserve-unused-selectors test to accompany this new option.

Related

• Addresses Unused css selector warning - disable specific parts? #1594, Add an option to prevent removal of unused CSS selectors/classes  #5804, and Change removal of unused CSS to just a warning #7751.
• Removal of unused selectors originally added in minify css and remove unused styles #729 in response to Minify CSS, and remove unused rules #697.

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
• Prefix your PR title with feat:, fix:, chore:, or docs:.
• 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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[Conduitry]

Did you happen to notice that all of the issues you linked to were closed as won't-implement?

[Maytha8]

What's the problem? It's a useful option that can greatly benefit those who develop with Svelte. And it's false by default.

[rkyoku]

@Maytha8 do not bother, this member is blind to users input. So many users agree with you though...