misc/wasm: add polyfill for TextEncoder/TextDecoder for Edge support

Edge supports WASM but not TextEncoder or TextDecoder.
This PR adds a polyfill to `misc/wasm/wasm_exec.js` to fix this.

Fixes #27295

Change-Id: Ie35ee5604529b170a5dc380eb286f71bdd691d3e
GitHub-Last-Rev: a587edae2806e1ca9b6be1c5dfd8824568373bdb
GitHub-Pull-Request: golang/go#27296
Reviewed-on: https://go-review.googlesource.com/131718


Reviewed-by: Agniva De Sarker <agniva.quicksilver@gmail.com>
Reviewed-by: Richard Musiol <neelance@gmail.com>