misc/wasm: enable ECMAScript strict mode

misc/wasm: enable ECMAScript strict mode

Current wasm_exec.js does not enable ECMAScript strict mode. But it is
recommended to be enabled because it

1. eliminates some ECMAScript silent errors by changing them to throw
   errors
2. fixes mistakes that make it difficult for JavaScript engines to
   perform optimizations
3. prohibits some syntax likely to be defined in future versions of
   ECMAScript

This commit enables ECMAScript strict mode in wasm_exec.js following the
transition guide:

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Strict_mode/Transitioning_to_strict_mode

Fixes #47116

Change-Id: Ib8ffceee37e9127698fb51304241f1e429efe83e
Reviewed-on: https://go-review.googlesource.com/c/go/+/334269
Reviewed-by: Richard Musiol <neelance@gmail.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Trust: Richard Musiol <neelance@gmail.com>
Run-TryBot: Cherry Mui <cherryyz@google.com>
TryBot-Result: Go Bot <gobot@golang.org>