[refactor] DRY out whitespace handling

[bayre]

Spurred on by a few outstanding issues (#3168, #7493) I've just been looking into how whitespace is handled in the Svelte compiler. It seems like there's a fair bit of repetition, with some functions being copied almost directly between render_dom and render_ssr. This PR is a bid to reconcile this in the hope that it'll be easier to maintain and easier to identify the source of any discrepancies between DOM and SSR outputs. That said, it's also my first foray into the Svelte source code so there may well be something I have overlooked!

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

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

[bayre]

Binning this for fear that it's unwarranted noise

[dummdidumm]

No worries! Refactorings like this are generally received well, it just may take some time until it's merged.