Use readonly types for arrays where appropriate

[evantrimboli]

Is your feature request related to a problem? Please describe.
It's not possible to pass readonly arrays for chart data, legends and other props that aren't mutated.

Describe the solution you'd like
Types should use readonly where appropriate:

export interface DataProps<RawDatum> {
  data: readonly RawDatum[];
}

Describe alternatives you've considered
N/A

Additional context
This has 2 benefits:

• It communicates intent better, so the caller can be sure the array itself isn't mutated
• For those using readonly arrays in the code, it makes it simpler because there's no need to shallow clone or cast.

[plouc]

That's indeed a really good idea, I had issues achieving this due to D3 typings, because D3 often mutates data, so the lib would have to handle what currently has to be done in user space.