Support new runtime `isJiti` and `isTsx`?

[KeJunMao]

Describe the feature

I'm developing a vite plugin using unbuild, if it's in stub mode(jiti), inject a package for main.ts using a relative path, otherwise use the package name.

Additional information

• Would you be willing to help implement this feature?

[KeJunMao]

If run directly as a command, I can use process.mainModule.filename to confirm the runtime.

If I use programming to load modules (like the stub mode of unbuild), I don't know how to determine the current runtime.

Currently, I am using the build: done hook of unbouild to output process.env.JITI=true'.

  hooks: {
    'build:done': (ctx) => {
      if (ctx.options.stub) {
        writeFileSync('dist/index.mjs', `process.env.JITI = true;\n${readFileSync('dist/index.mjs', 'utf-8')}`)
      }
    },
  },

[pi0]

I think we might need to expose such env/static flag from jiti itself to allow detection.