gen-host-js: instance ESM output by default (#406)

* gen-host-js: instance ESM output by default

Outputs a direct ES module from the JS host generator by default
that can be directly imported in JS environments.

For the custom instantiation API that was the previous output, a new
`--instantiation` flag is provided.

Instance-level customization flags are also added including:
* `--compat`: Enables compatibility mode, based on the assumption
  that a new --tla-compat flag will be added in a future PR.
* `--nodejs-compat`: Handle Node.js instantiation when no fetch global
  is present

The above flags are only compatible with instance output and will throw
otherwise.

This PR also parallelizes the core Wasm instantiations, and converts the 
instantiation API from `instantiateCore(path, imports)` to
`compileCore(path)` to enable this parallelization.

In addition a new `--map` is added with support for custom import
specifier mappings that can apply to both output modes.

Author: Guy Bedford

Cargo.lock

@@ -6,7 +6,7 @@ module.exports = {
     },
     "extends": "eslint:recommended",
     "parserOptions": {
-        "ecmaVersion": 12,
+        "ecmaVersion": 13,
         "sourceType": "module"
     },
     "rules": {

crates/gen-host-js/.eslintrc.js

@@ -10,6 +10,7 @@ test = false
 
 [dependencies]
 wit-bindgen-core = { workspace = true, features = ['component-generator'] }
+anyhow = { workspace = true }
 heck = { workspace = true }
 clap = { workspace = true, optional = true }
 wasmtime-environ = { workspace = true, features = ['component-model'] }

crates/gen-host-js/Cargo.toml

@@ -1,9 +1,9 @@
 {
   "devDependencies": {
     "@types/node": "^15.12.2",
-    "@typescript-eslint/eslint-plugin": "^4.27.0",
-    "@typescript-eslint/parser": "^4.27.0",
-    "eslint": "^7.28.0",
+    "@typescript-eslint/eslint-plugin": "^5.41.0",
+    "@typescript-eslint/parser": "^5.41.0",
+    "eslint": "^8.26.0",
     "typescript": "^4.3.2"
   }
 }