docs: TypeScript Support & Example

[jthegedus]

Describe the bug

TypeScript may cause issues because our SvelteKit app is already built when the build output lands in functions/lib/<site name>/.

How should this be recommended to users? If they include the compiled relative import in the uncompiled TS then the app may attempt to be recompiled by the TS compilation of the functions, which we do not want :thinking:

EG:

// functions/src/index.ts
svelteKit = require("../lib/<site name>/handler").sveltekitServer;` 

This relative dir will be correct in the compiled code in functions/lib/index.js, but if the SvelteKit code is also passed through the TS compiler then we may have issues.

[joefhall]

+1 for this -- would be great to have easy integration with Typescript