[web] revise flag `ort.env.wasm.simd` (#24314)
### 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
#24292 (@Eldow)
- Allow setting to `'relaxed'` to enable Relaxed SIMD checking. Relaxed
SIMD is introduced first in #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
