This commit adds support for native system access for WASM. This is used
through a new `wasmSystemAccess` option on the `esbuild.initialize`
options bag. This options bag can contain either a specifier or
namespace for a Node.js compatible `fs` and `process` module. During
setup these are injected into `globalThis.fs` and `globalThis.process`,
where Go's WASM runtime uses them to back the regular Go FS APIs.
Finally, if this option is set, we set the `hasFS` option on
`createChannel`, and everything just works :)

Example for Deno:

```ts
import * as esbuild from "./deno/wasm.js"

await esbuild.initialize({ wasmSystemAccess: {
  fsSpecifier: "node:fs",
  processSpecifier: "node:process"
} })

await esbuild.build({
  format: "esm",
  entryPoints: ["./lib/deno/mod.ts"],
  target: "esnext",
  outfile: "./out.js"
})
```