Add readonly support

[mattroberts297]

Because I wanted to enforce some immutability

PS Thanks for the awesome library / tool

[jameswilddev]

This looks like it would resolve #131, excepting the use of ReadOnlyArray. Do you know if it would also turn { [k: string]: T } into { readonly [k: string]: T}?

[bcherny]

Looks good, but I'd love if you could iterate on this a bit more before we merge it (also, sorry for the delay in getting to this!).

1. Please rename the option to --readonly.
2. Could you make sure it also makes index signatures (as @jameswilddev noted), array types, tuple types, and referenced types readonly?
3. Please also update README.md to document this.

[bcherny]

Happy to take a look once you have a chance to address the above feedback.

[jonaskello]

I'm also looking for this function, is someone still working on it?

[mattroberts297]

I stopped working on this PR because I moved away from the language (as opposed to the library) for a time. Thanks for the feedback and apologies for not addressing it.

[codingismy11to7]

I'm guessing this PR will probably get closed, but just wanted to chime in that I'd additionally expect

export type X = Base & {
  field: type
};

to output as

export type X = Base & Readonly<{
  field: type
}>;

which this code doesn't seem to address

[bcherny]

#412