[Feature] UMD bundle support

[UMD](https://github.com/umdjs/umd) is used by many libraries to support legacy module systems. Migrating to esbuild is difficult when relying on this format.

There are some workarounds but they have major downsides:
1. Post-process the esbuild IIFE output to add the UMD wrapper if you don't have any external dependencies: Makes sourcemap difficult to deal with and makes builds slower. (#482) would simplify this workaround and solve the source map issue but still not allow external dependencies.
2. Use rollup/webpack's UMD support with esbuild transformers (instead of babel): Doesn't benefit from esbuild's ultra fast dependency resolution and bundling.

Willing to give this a go myself if there is interest to include this feature.

No file, test, or entry point is named in the issue. Start by tracing esbuild's existing output-format handling and reviewing UMD requirements alongside the listed workarounds; done means esbuild can emit UMD bundles, including with external dependencies, without the cited source-map and performance drawbacks.

Written by the indexing model from the issue text.