[feat] explicitly set compilerOptions.hydratable to config.kit.hydrate

[dominikg]

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 pnpx changeset and following the prompts. All changesets should be patch until SvelteKit 1.0

Until now vite-plugin-svelte defaults to hydratable: true, but that is going to change.
See sveltejs/vite-plugin-svelte#122

To avoid breaking, kit has to set hydratable explicitly.
vite-plugin-svelte also offers a new option experimental.dynamicCompileOptions that could be used to implement a more finegrained solution of which components need hydration. see sveltejs/vite-plugin-svelte#121

[changeset-bot]

🦋 Changeset detected

Latest commit: 849e635

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

[benmccann]

lint is failing

[dominikg]

lint is failing

hmm, locally both pnpm lint and pnpm check passed without failing.
svelte-check output mentioned a config load issue though for pnpm check

│ Error while loading config
│ TypeError: Cannot read property 'compilerOptions' of undefined
│     at ConfigLoader.loadConfig (/home/dominikg/develop/sveltejs/kit/node_modules/.pnpm/svelte-check@2.2.0_svelte@3.40.0/node_modules/svelte-check/dist/src/index.js:26654:31)
│     at async ConfigLoader.loadAndCacheConfig (/home/dominikg/develop/sveltejs/kit/node_modules/.pnpm/svelte-check@2.2.0_svelte@3.40.0/node_modules/svelte-check/dist/src/index.js:26643:46)
│     at async /home/dominikg/develop/sveltejs/kit/node_modules/.pnpm/svelte-check@2.2.0_svelte@3.40.0/node_modules/svelte-check/dist/src/index.js:26605:17
│     at async Promise.all (index 2)
│     at async ConfigLoader.loadConfigs (/home/dominikg/develop/sveltejs/kit/node_modules/.pnpm/svelte-check@2.2.0_svelte@3.40.0/node_modules/svelte-check/dist/src/index.js:26607:13)
│     at async createLanguageService (/home/dominikg/develop/sveltejs/kit/node_modules/.pnpm/svelte-check@2.2.0_svelte@3.40.0/node_modules/svelte-check/dist/src/index.js:79394:5)

[benmccann]

lint is passing now. the warning is a bug in svelte-check