feat: replace flattenArray with build-in Array.flat function

[tkgroot]

replacing the helper flattenArray with build-in Array.flat function

[Pulse-Git]

@tkgroot Array.flat is not supported in IE11 or am i wrong?
This lib is supported by IE11 so, maybe an check is required here

[tkgroot]

No, i don't think you are wrong.
Maybe a good idea to drop IE11 support or let it be handled by polyfills?
Also if you look ahead IE11 doesn't support ESM, so it's just a big hurdle to keep the support for it