Toggle JSX namespace based on svelte vs svelte-native

[halfnelson]

This brings Svelte Native support to language tools.
Since svelte and svelte-native have different intrinsic elements, it was essential to namespace them (JSX -> svelte.JSX)
The namespace is chosen by typescript based on the jsxFactory compiler option. I have changed this to be either svelte.createElement or svelteNative.createElement to select either svelte.JSX or svelteNative.JSX namespace.

The code tries to detect svelte-native by resolving the package. The namespace can be forced by providing jsxFactory compiler option in your tsconfig.

I had to change a couple of tests to handle the new test folder I added, and to handle the namespace change.
I think the language server will work with old versions of svelte2tsx (as the namespace falls back to JSX) but an old language-server release against a new svelte2tsx will probably show unwanted TS errors

[dummdidumm]

Great stuff! Nice side effect: This also fixes #228, no more clashes with react types because it's now regarded as a different factory.