React 18: Separate `concurrentFeatures` and `runtime` options

[devknoll]

• Add the global experimental runtime flag to switch the default runtime. If runtime is set, we enable Fizz for all pages. It works similar to the concurrentFeatures flag. If runtime is missing, Fizz will not be enabled.
  • Add support for nodejs runtime. (@shuding)
• Remove concurrentFeatures from the config but keep it internal. It will be disabled for now.
• Add per-page runtime flag to enable Fizz for specific pages. The global runtime will now not affect all pages.
  • Pre-parse each page before any compilation happens to collect page configurations. (@huozhi)
  • Generate entries for each compilers based on page and global runtimes. (@shuding)
• Allow the global runtime to be unset. (@shuding)

[jlarmstrongiv]

Separating the concurrent and runtime features would be great, as it’s trivial to hit the 1MB limit of the vercel edge runtime #32907 and break your app