Add CommonJS support as a plugin option

[timostamm]

This PR adds support for generating JavaScript for the CommonJS module system.

When the plugin option js_import_style is set to legacy_commonjs, generated JavaScript will use require() to import, and the exports variable to export.

Here is a shortened example, generated from example.proto:

// @generated by protoc-gen-es v1.5.1 with parameter "target=js+dts,js_import_style=legacy_commonjs"
// @generated from file extra/example.proto (package docs, syntax proto3)

"use strict";
Object.defineProperty(exports, "__esModule", { value: true });

const { proto3 } = require("@bufbuild/protobuf");

/**
 * @generated from message docs.User
 */
const User = ...

exports.User = User;

As you can see, strict mode is enabled. For best interoperability, we define __esModule.

For plugin authors:

• To support CommonJS output, your code must use exportDecl instead of printing an export statement. For example, use f.print(f.exportDecl("abstract class", "MyClass"), " {}") instead of f.print("export abstract class MyClass {}").
• In case you are overriding the built-in transpilation, your transpile function needs to handle the new 4th argument jsImportStyle to support CommonJS output.

Closes #587.

[smaye81]

Do you also want to update the docs here? https://github.com/bufbuild/protobuf-es/blob/main/docs/generated_code.md?plain=1#L81-L86

[timostamm]

Do you also want to update the docs here? https://github.com/bufbuild/protobuf-es/blob/main/docs/generated_code.md?plain=1#L81-L86

Thanks! Updated in 526fe50.

[mfridman]

🎉