support result in types generation

[ctaggart]

In changes my WIT definition from:

create-program: func(options: compiler-options) -> program;

to return a result:

create-program: func(options: compiler-options) -> result<program, string>;

but the generated .d.ts from bunx jco types wit -o . still has:

export function createProgram(options: CompilerOptions): Program;

I was hoping that it would show up as:

export function createProgram(options: CompilerOptions): Result<Program, string>;

I'm not exactly sure where Result would be defined, but I see a it defined a couple of times in preview-2shim as:

export type Result<T, E> = { tag: 'ok', val: T } | { tag: 'err', val: E };

[guybedford]

Just like top-level optionals become possibly nullish, top-level results utilize the JS error itself.

So if you want to create an error result you can do throw 'error-string'.

We could possibly have a customization option to turn this off, but it's also useful to be able to interop with JS error handling paths.

We should also have proper documentation of the type conversions like this, this is still tracking in #418.