Ability to disable code-splitting for multi-entrypoint builds

[Xerios]

One of the issues I've encountered while using the new experimental optimize feature is that the bundled build tends to create a shared file which is used by all entrypoints that require it.

This isn't a bad thing, but in my case, where I'm bundling one main.js file and a webworker, the webworker should be self-contained.
Adding imports breaks FireFox compatibility since they don't support modules yet ( it gives the error: import declarations may only appear at top level of a module worker )

Setting splitting: false inside optimize.ts fixes my issue, and avoids including any imports inside of my webworker.

Perhaps this should be a setting alongside with other optimize options?

[FredKSchott]

Yea, we've been going back and forth on whether we ship splitting on/off by default. I don't think it'll be ready in esbuild by the time that we launch so we'll most likely ship off by default and then add an option to enable.

@Xerios if you're interested in changing this default behavior and adding the new splitting option, we'd love a PR!