fix: use ReadonlyArray type for table parameter / align option

[bmish]

This should be a backwards-compatible improvement to the markdownTable function parameter/option TypeScript types. In addition to allowing regular (writable/mutable) arrays, the user can now provide read-only arrays. This is possible because markdown-table does not modify these input arrays anyway.

Without this change, someone wanting to pass a read-only array to this function would have to first recreate the array (like [...arr]) in order to do so.

Related documentation about the readonly arrays in TypeScript:

• https://mariusschulz.com/blog/read-only-array-and-tuple-types-in-typescript#read-only-array-types-in-typescript
• https://www.typescriptlang.org/docs/handbook/release-notes/typescript-3-4.html#a-new-syntax-for-readonlyarray

[remcohaszing]

I think this makes sense for options / arguments

[wooorm]

Thanks, released in 3.0.3!