Separate build + transpilation stages

[karimsa]

For debugging purposes, minimal transpilation is very useful. I think the best way to achieve this would be to separate the build and transpiling phases, so that we initially cache and build the app with ESNext as the target (i.e. no transpiling).

Then when we serve the app, run it back through esbuild to perform transpiling and try to target the consuming browser. We can parse the user agent string and transform that to an esbuild target (ref for parsing). If we fail to parse, we can always target something like ES2015 which has really wide support.