Don't use esbuild for building framework

[zardoy]

I'm currently using esbuild for building all packages. The main reason is not speed, but ability to use __DEV__ env variable and strip development code in production.

Unfortunately, I just figured out that esbuild isn't good for building libraries, since, even in cjs format it doesn't allow to mock any exports, which is blocking testing at any position and it is high priority for me. evanw/esbuild#412 (comment)

Currently I don't use environment variables, so I try to stick with the TypeScript compiler

[zardoy]

esbuild.mjs