enable using esbuild with deno and wasm

[evanw]

The native implementation of esbuild is much faster than the WebAssembly version, but some people don't want to give Deno the --allow-run permission necessary to run esbuild and are ok waiting longer for their builds to finish when using the WebAssembly backend. With this PR, you can now use esbuild via WebAssembly in Deno. To do this you will need to import from wasm.js instead of mod.js:

import * as esbuild from 'https://deno.land/x/esbuild@v0.14.48/wasm.js'
const ts = 'let test: boolean = true'
const result = await esbuild.transform(ts, { loader: 'ts' })
console.log('result:', result)

Make sure you run Deno with --allow-net so esbuild can download the WebAssembly module. Using esbuild like this starts up a worker thread that runs esbuild in parallel (unless you call esbuild.initialize({ worker: false }) to tell esbuild to run on the main thread). If you want to, you can call esbuild.stop() to terminate the worker if you won't be using esbuild anymore and you want to reclaim the memory.

Note that Deno appears to have a bug where background WebAssembly optimization can prevent the process from exiting for many seconds. If you are trying to use Deno and WebAssembly to run esbuild quickly, you may need to manually call Deno.exit(0) after your code has finished running.

Fixes #2323