Use escape sequences to denote special characters

[Rich-Harris]

The latest chapter in the encoding saga

closes #7621
closes #7604
closes #7554
closes #7567
closes #7573
closes #7563
closes #7577

TODO:

• documentation
• allow escape sequence next to a parameter
• validate escape sequences
• test surrogate pairs on second thoughts we don't need to use surrogate pairs, we can just support [u+10ffff] directly

Please don't delete this checklist! 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
• 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 pnpm test and lint the project with pnpm lint and pnpm check

Changesets

• If your PR makes a change that should be noted in one or more packages' changelogs, generate a changeset by running pnpm changeset and following the prompts. All changesets should be patch until SvelteKit 1.0

[changeset-bot]

🦋 Changeset detected

Latest commit: c697df7

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@sveltejs/kit	Patch

Not sure what this means? Click here to learn what changesets are.

Click here if you're a maintainer who wants to add another changeset to this PR