* feat(config): implement compiler.define and compiler.defineServer (#1496)
Forward `compiler.define` to Vite's top-level `define` (applies to both
client and server bundles) and `compiler.defineServer` to non-client
environments only via a `configEnvironment` hook, matching Next.js
Webpack `DefinePlugin` semantics. Values are JSON-stringified at config
resolution time so they slot directly into Vite's substitution format.
Refs Next.js: packages/next/src/build/define-env.ts.
Fixes#1496
* fix(config): reject compiler.define keys that collide with built-ins
Mirrors Next.js (`packages/next/src/build/define-env.ts`) — throw when a
user-provided `compiler.define` or `compiler.defineServer` key would
silently overwrite an internal define (e.g. `process.env.NODE_ENV`,
`__NEXT_*`, `__VINEXT_*`) or when `define` and `defineServer` define the
same key. Without this, a misconfigured project would produce subtly
wrong output instead of failing fast.
Addresses /bigbonk review on #1638.
James Anderson