Replace __OXIDE__ at build time to prevent @tailwindcss/oxide leaks in the stable engine

[RobinMalfait]

This PR will ensure that the oxide engine doesn't leak in the current stable version.

The oxide engine is currently behind a runtime feature flag:

if (env.OXIDE) {
  require('@tailwindcss/oxide').doSomethingOxideSpecific()
}

But some build tools that use the commonjs plugin (often used in Vite / Rollup) will hoist that require part out of the guard... this typically results in a static import instead. But since the stable engine doesn't have any idea what @tailwindcss/oxide even is, it crashes.

If you would inspect the generated code, then it will look something like this:

import moduleName from '@tailwindcss/oxide'

if (env.OXIDE) {
  moduleName.doSomethingOxideSpecific()
}

... which kind of defeats the whole purpose of the feature flag in the first place. In a project where OXIDE is defined this all works fine, but in other places it won't.

This PR will switch the runtime feature flag, to a build time feature flag. This will ensure that the code is not even there in the final built code but that should be fine since you can switch between the engines if you install a different version (insiders vs oxide-insiders).

We will replace __OXIDE__ at build time. This also means that some parts of the code will now look something like if (false) { require('@tailwindcss/oxide') }. This means that the block will never be reachable, but luckily SWCs optimizer will get rid of the dead code entirely.

[thecrypticace]

LGTM