allow passing in context in constructor

[tanhauhau]

This allows passing in context through the Svelte Component Constructor API

import Component from './Component.svelte';

const context = new Map();
map.set('a', 123);
map.set('b', 456);

new Component({
   target: document.body,
   context: context,
});

Before submitting the PR, please make sure you do the following

• 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

[Conduitry]

Do we want a way to do this for SSR components as well?

[tanhauhau]

That make sense, let me add for SSR components as well.

[Conduitry]

The new SSR test is failing, which I can also reproduce locally. I'm not sure what's going on there. It does pass if I add solo: true to it to run only that test.