Encoding value casing (eg. ASCII)
Currently the JS runtime only allows the `encoding` value to be `ASCII`, but not `ascii`:

https://github.com/kaitai-io/kaitai_struct_javascript_runtime/blob/8fedb4d2f278f55d0d8fcf24a9bbf1c1ea2b150f/KaitaiStream.js#L511-L513

I checked the formats repo and 9 file formats used lowercase `ascii` at 23 occurrences.

Should we fix the formats or should the runtimes accept lowercase `ascii` too?

(I presume we should fix the runtimes, but maybe add a KSY styling rule which casing should be used generally?)