feat(ts): set `columns` type as `readonly`

[blemoine]

The goal is for stringify to support values declared as const.

eg.

const columns = ['name', 'age'] as const

stringify({columns})

As this array has no reasons to be mutated by stringify it's simpler to set it as readonly.

[wdavidw]

This is pretty much the case for every option. Not sure it provide much value.

[blemoine]

If you try the following code

const columns = ['name', 'age'] as const

stringify({columns})

without this PR patch, it won't compile - even though it's perfectly valid at runtime.

[ViniciusLovato]

Recently, I encountered the same problem. It would be helpful to support values declared as const.

[wdavidw]

Then I propose to update this PR to apply const to every option.

[yoursunny]

The Input type should be readonly too.