Live viewer: switch from browserify to esbuild

Browserify is practically unmaintained, and requires a lot of dependencies. esbuild is a more modern alternative written in Go that is better on all those aspects.

This also enables source maps for the bundle while we're at it.

Author: TimothyGu

.gitignore

@@ -6,6 +6,7 @@ npm-debug.log
 coverage/
 test/web-platform-tests/
 live-viewer/whatwg-url.js
+live-viewer/whatwg-url.js.map
 lib/VoidFunction.js
 lib/Function.js
 lib/URL.js