feat: allow use of createContext when instantiating components programmatically

[Rich-Harris]

This enables an advanced pattern when creating type-safe context with createContext — you can pass a wrapper component to mount or hydrate or render that calls the setter before calling the component. The only reason this was previously impossible was because a component context wasn't automatically created for the root, unless context was provided.

It seems likely that we'll want to deprecate setContext, getContext, hasContext and the context option in the future, since createContext is a superior approach. This PR doesn't do anything in that direction though — we can consider that separately.

Alternative to #17515, closes #16997.

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.
• If this PR changes code within packages/svelte/src, add a changeset (npx changeset).

Tests and linting

• Run the tests with pnpm test and lint the project with pnpm lint

[changeset-bot]

🦋 Changeset detected

Latest commit: c4bb814

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

This PR includes changesets to release 1 package

Name	Type
svelte	Minor

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

[svelte-docs-bot]

Preview: https://svelte-dev-git-preview-svelte-17575-svelte.vercel.app/

[github-actions]

Playground

pnpm add https://pkg.pr.new/svelte@17575

[brunnerh]

Regarding deprecations/removals: The context option commonly is still useful to pass all existing contexts along via getAllContexts.