Experimental support for Deno

[evanw]

This PR introduces experimental support for Deno. You can build the Deno esbuild package using make platform-deno which generates two files, ./deno/mod.js and ./deno/mod.d.ts. When run, mod.js will download and cache the esbuild binary from npm so you'll need network access to npm to use this package. This package requires the --allow-env --allow-read --allow-write --allow-net --allow-run Deno permissions.

The API for the Deno esbuild package is the same as the npm esbuild package with one exception: you need to call stop() when you're done, which kills esbuild's child process. It doesn't look like Deno has an equivalent to node's unref() API so I couldn't figure out how to allow Deno to exit while the esbuild child process is still running. Maybe it's not possible to do that in Deno?

Issue: #936