[web] revise flag ort.env.wasm.simd

[fs-eire]

Description

This PR revised the flag ort.env.wasm.simd to enhance its usage so that more use scenarios are covered.

• Allow setting to false explicitly to disable SIMD checking. resolves [Web] Allow to disable the SIMD detection check #24292 (@Eldow)
• Allow setting to 'relaxed' to enable Relaxed SIMD checking. Relaxed SIMD is introduced first in [js/web] Add Wasm Relaxed SIMD support to wasm backend #22794 (@jing-bao)
• Behavior is not changed when not setting (ie. undefined) or setting to true
• Added a warning message when setting to unknown value, and reset to false in this case