Export createAdapter to make writing custom adapters easier

[magicalpuffin]

Is your feature request related to a problem? Please describe.

I was having issues with the typebox adapter because the environment (cloudflare pages) doesn't allow type compiling. Trying to validate would result in a code generation error. To work around this, I wanted to use createAdapter and implement validate without compiling, however, it isn't exported. I ended up having to patch sveltekit-superforms/adapters/index.js.

sveltekit-superforms/src/lib/adapters/typebox.ts

Line 25 in eabcb20

async function validate<T extends TSchema>(

Describe the solution you'd like

Export createAdapter and other useful types to make it easier to implement custom adapters.

https://github.com/ciscoheat/sveltekit-superforms/blob/main/src/lib/adapters/index.ts

It would be nice to have a version of the typebox adapter without compiling, maybe a typeboxNoCompile. I do realize it might be niche, so exposing createAdapter with some documentation about writing adapters would be a more general solution.

[ciscoheat]

Hello, thank you for the PR, the reason I'm hesitant is that as soon as something is exported, I have to support it and prevent breaking changes, and the createAdapter interface isn't set in stone. I was thinking about doing it in v3, so I'll add it to that milestone for the time being.