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`.

 - csv-demo-browser@0.1.19
 - csv-demo-cjs@0.2.17
 - csv-demo-eslint@0.1.21
 - csv-demo-esm@0.0.31
 - csv-issues-cjs@0.2.6
 - csv-issues-esm@0.3.0
 - csv-demo-ts-cjs-node16@0.2.17
 - csv-demo-ts-module-node16@0.2.17
 - csv-demo-webpack-ts@0.1.19
 - csv-demo-webpack@0.1.21
 - csv-generate@4.4.1
 - csv-parse@5.5.6
 - csv-stringify@6.5.0
 - csv@6.3.9
 - stream-transform@3.3.2