svelte2tsx utils and declarations map

[dummdidumm]

With the change to a named export and the experience of transitioning quite a lot code in #1070, I think it's time to introduce

• some utilities related to svelte2tsx which right now are scattered in other language tools repos
• a generated declaration map similar to svelteShims.ts where we generate a map of all declared variables/functions/types in svelte-shims.d.ts . Those can then be used in all places where they are used to improve code navigation and ease refactorings and version bumps to these declarations