Generate exact JS buffer types for exact IDL buffer arguments instead
of widening them to BufferSource or ArrayBufferView.
This fixes cases like TextEncoder.encodeInto(), whose IDL requires
a Uint8Array destination. Previously the generated binding accepted
any BufferSource, so DataView, other typed arrays, and
ArrayBuffer-backed values were let through. With exact conversion,
those are rejected at the binding layer as expected.
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