[WIP] new module jsarrays: ArrayBuffer, DataView + typed js API's

[timotheecour]

Do not review but early feedback welcome.

This PR adds a module jsarrays.nim, wrapping ArrayBuffer, DataView + typed js API's, see https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/DataView
This is useful for cases where the 1-size-fits-all js type Number falls short.

This PR will allow #14095 to have streams.read[T] (and write[T]) with T integer or FP types
(right now #14095 only supports T string)

This PR will be cleaned up to remove the commits from #14095 and only introduce jsarrays module + tests, but right now they're included to show this works with streams, see test tests/js/tstreams.nim

#14095 (or a followup PR) can thus use jsarrays to support read/write[T]

BigInt (for nim-lang/RFCs#187) could belong here (see https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/DataView/getBigInt64) or be introduced in another module and imported here.

note

• ArrayBuffer + DataView can be used to implement string in js more efficiently: right now it uses Array (which is dynamically typed), but with ArrayBuffer + DataView it would be backed by a strongly typed array of char.
• this should either fix or help Casting of pointers in JS backend #5667 once completed

EDIT

merge with lib/std/private/jsutils.nim

[RSDuck]

are typed arrays such as Uint8Array planned as well?

[stale]

This pull request has been automatically marked as stale because it has not had recent activity. If you think it is still a valid PR, please rebase it on the latest devel; otherwise it will be closed. Thank you for your contributions.