[Feature] Support dynamic imports via plugins

I know there have been multiple threads on the subject (#661, #453, #322, #56), but since plugins are now a thing I figured it could be worth opening one more discussion. Perhaps there could be a solution that doesn't require statically pregenerating files? At least I haven't seen my suggestion elsewhere 😄

ESBuild currently warns when it detects dynamic imports, and leaves them in-place. It's a good start, but it's not enough: whatever is tasked from consuming them will still lose critical information, in particular the location the dynamic import is relative to (for instance, when doing an `import('./widgets/' + name)` into an application compiled via `--bundle`, I really need to know where was the file that made this import).

My suggestion to support dynamic imports with only a little extra API surface would be to instead yield the generation of an intermediary dynamic import module to a special type of loader. For example, the following loader would allow dynamic imports where each module would end up in their own chunk:

```ts
// `dynamic` would be a builtin namespace used only for dynamic imports
build.onLoad({ namespace: `dynamic` }, async args => {
// Similar to how template strings work; args.path
// would be a special path generated by esbuild
const [importer, segments, ...expressions] = JSON.parse(args.path);

// Would find all the files that match the path
const files = glob(segments.join(`**`));

// Then generates the intermediary file
return {
// Note that this assumes that esbuild would then transform something
// like `import('foo')` into `import('')('foo')`
contents: `
export default async function (p) {
switch (p) {
${files.map(file => `
case "${file}": return import("${file}");
`).join(``)}
}
}
`,
};
});
```

On the other hand, if I wanted to make a dynamic import where all modules are in the same chunk, then I'd just need my loader to generate regular `import from` statements, without the consumer code having to know about this. I feel like this might match the kind of flexibility you had in mind given how loaders currently work.