[WebNN] Improve data type check of slice op (#22988)
A follow-up of [[WebNN] Support negative steps for
slice](https://github.com/microsoft/onnxruntime/pull/22871#discussion_r1847929774).
Slice op is emulated by reverse+slice when steps < 0 so
`SliceOpBuilder::HasSupportedInputsImpl()` should also check the
supported data types of reverse.
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Co-authored-by: Wanming Lin <wanming.lin@intel.com>
